K4V

Garrison94 does not list a K4V anchor standard, and MK73 does not list
a K4V dagger standard. Only two stars in the Bright Star Catalog (5th ed.)
are classified as K4V (HR 5568 = HD 131977, HR 8721 = HD 216803). 

The primary (GJ 689 = HD 160964), secondary (GJ 879 = HD 216803 = TW
PsA), and tertiary (GJ 570A = HD 131977) standards are all near solar,
so the ~solar composition mean properties for K4V are well-defined.

Morgan65 classified two Hyads as K4V: HD 285720 & HD 285742. Pecaut16
finds that Houk/Michigan stars classified as "K3/4V" map well to "K4V"
among the classifications of Gray03/06, and Houk/Michigan "K4V" maps
roughly to "K4+V" on the Gray03/06 (note the Gray classifications use
the Keenan standards).

Keenan80 standard:   Gl 689 (=HD 160964) - K4V
Keenan83 standard:   Gl 689 (=HD 160964) - K4V
Keenan85 standards:  Gl 689 (=HD 160964) - K4V
	 	     Gl 879 (=HD 216803 = TW PsA) - K4V 
Keenan88 standard:   Gl 689 (=HD 160964) - K4V
Keenan89 standard:   Gl 689 (=HD 160964) - K4V
GrayNstars standard: GJ 570A (=HD 131977) - K4V
Pecaut13 standard:   Gl 879 (=HD 216803 = TW PsA) - K4V

Skiff08 flags one star as K4V MK standard: HD 160964 (Keenan89).

The mean B-V for Hipparcos K4V stars is 1.115. So its not clear that
HD 131977 is representative either.

<B-V>(K4V) =  1.168 [+-0.033 sem,+-0.140 stdev]; med. for 28 K4V stars ARICNS
(B-V)(K4V) =  1.15  ; exemplar 70 Oph B
(B-V)(K4V) =  1.141 ; exemplar GJ 775
(B-V)(K4V) =  1.137 ; exemplar HD 105671
(B-V)(K4V) =  1.130 ; median SIMBAD K4V d<25pc (N=48)
(B-V)(K4V) =  1.128 ; median Koen10 d<25pc (N=15)
(B-V)(K4V) =  1.115 ; median for N=41 K4V stars w/i 30pc (Hinkel17)
(B-V)(K4V) =  1.115 ; exemplar HD 24916
<B-V>(K4V) =  1.111 ; 58 HIP d<75pc (+-0.009 sem, +-0.084 stdev)
<B-V>(K4V) =  1.101 ; 46 Gray01/03/06 (+-0.006 sem, +-0.042 stdev)
<B-V>(K4V) =  1.10  ; Hyades; Wilson62(N=11)
(B-V)(K4V) = 1.099 ; interpolation for standards & exemplars for [Fe/H]=0.0 (B-V=1.099-0.072*[Fe/H], rms=0.045)
(B-V)(K4V) = 1.094 ; sec. stan. GJ 879/TW PsA
(B-V)(K4V) = 1.089 ; pri. stan. GJ 689
(B-V)(K4V) = 1.084 ; trend for xHIP K4V d<30pc w/[Fe/H] values with uncertainties 
(B-V)(K4V) =  1.060 ; exemplar GJ 845 (eps Ind)
(B-V)(K4V) =  1.05  ; exemplar HD 285720 (Hyad, metal-rich)
(B-V)(K4V) =   1.05  ; Schmidt-Kaler82
(B-V)(K4V) =  1.03  ; exemplar HD 285742 (Hyad, metal-rich)
(B-V)(K4V) = 1.024 ; ter. stan. GJ 570A/HD 131977 
<B-V>(K4V) =   1.02  ; Bessell79
<B-V>(K4V) =   1.00  ; Fitzgerald70
=> adopt (B-V)(K4V)  = 1.090 *** [updated 12/15/2020]

=> adopt (U-B)(K4V)  = 0.986
=> adopt (V-I)(K4V)  = 1.182
=> adopt (V-R)(K4V)  = 0.635
=> adopt (R-I)(K4V)  = 0.547
=> adopt (V-J)(K4V)  = 2.040
=> adopt (V-H)(K4V)  = 2.580
=> adopt (H-Ks)(K4V) = 0.123
=> adopt (J-H)(K4V)  = 0.539

(Ks-W1)(K4V) = 0.087 ; EEM trend for d<30pc for B-V=1.10
=> adopt (Ks-W1)(K4V) = 0.087 [updated 12/29/2019]

M_G(K4V) = 6.656 ; SIMBAD d<20pc trend for V-Ks=2.70

(G-V)(K4V)  = -0.4558 ; GJ_689
(G-V)(K4V)  = -0.4339 ; GJ_570A ter. stan.
(G-V)(K4V)  = -0.426  ; GJ_689 pri. stan.
(G-V)(K4V)  = -0.4189 ; GJ_879 sec. stan. 
(G-V)(K4V)  = -0.404  ; color fit for B-V=1.09
(G-V)(K4V)  = -0.3995 ; GJ_689 pri. stan. G_GaiaEDR3
(G-V)(K4V)  = -0.394  ; trend for SIMBAD d<20pc for V-Ks=2.70
(G-V)(K4V)  = -0.379  ; smooth fit f(SpT) G9V-M0V
=> adopt (G-V)(K4V) = -0.42 [updated 11/1/2020]
[note 9/21/2021: G-V colors seem noticeably off against overall trend for K subtypes, as if there
were an unusually large gap from K3.5V to K4V and K4V being not far from K4.5V and K5V.
Jump between G-V ~ -0.36 and -0.40 among standards and exemplars. This is a bit surprising as all
three standards are ~solar metallicity?]

(Bp-Rp)(K4V) =  1.2577 ; HD_285720
(Bp-Rp)(K4V) =  1.2835 ; *_eps_Ind   K4V(k)
(Bp-Rp)(K4V) = 1.3188 ; HD_131977   K4V ter. stan.
(Bp-Rp)(K4V) = 1.3260 ; V*_TW_PsA   K4Ve sec. stan.
(Bp-Rp)(K4V) = 1.345  ; color fit for B-V=1.09
(Bp-Rp)(K4V) = 1.3493 ; GJ 689 K4V pri. stan.
(Bp-Rp)(K4V) =  1.3509 ; GJ_9391A
(Bp-Rp)(K4V) =  1.3534 ; GJ_775
(Bp-Rp)(K4V) =  1.4773 ; *_70_Oph_B  K4V
=> adopt (Bp-Rp)(K4V) = 1.34 [last updated 11/10/2020]

(Bp-G)(K4V) =  0.5764  ; HD_285720
(Bp-G)(K4V) = 0.6290  ; GJ_879 sec. stan.
(Bp-G)(K4V) = 0.6293  ; GJ_689 pri. stan.
(Bp-G)(K4V) = 0.63   ; Bp-Rp=1.34, G-Rp=0.71
(Bp-G)(K4V) =  0.6332  ; GJ_775
(Bp-G)(K4V) =  0.6357  ; GJ_9391A
(Bp-G)(K4V) = 0.6535  ; GJ_570A ter. stan.
(Bp-G)(K4V) =  0.7273  ; GJ_702B
=> adopt (Bp-G)(K4V) = 0.63 [last updated 11/10/2020; Bp-Rp=1.34, G-Rp=0.71]

(G-Rp)(K4V) =  0.6653  ; GJ_570A
(G-Rp)(K4V) =  0.6813  ; HD_285720
(G-Rp)(K4V) = 0.669   ; color fit for B-V=1.09
(G-Rp)(K4V) = 0.6930  ; HD_131977   K4V ter. stan.
(G-Rp)(K4V) =  0.6970  ; GJ_879
(G-Rp)(K4V) = 0.7043  ; V*_TW_PsA   K4Ve sec. stan.
(G-Rp)(K4V) =  0.7070  ; *_eps_Ind   K4V(k)
(G-Rp)(K4V) = 0.71   ; Bp-Rp=1.34, Bp-G=0.63 
(G-Rp)(K4V) =  0.7153  ; GJ_9391A
(G-Rp)(K4V) = 0.7200  ; GJ 689 K4V pri. stan.
(G-Rp)(K4V) =  0.7203  ; GJ_775
(G-Rp)(K4V) =  0.7500  ; GJ_702B
(G-Rp)(K4V) =  0.7717  ; *_70_Oph_B  K4V
=> adopt (G-Rp)(K4V) = 0.71 [last updated 11/10/2020]

(U-B)(K4V)  = 1.013 ; HIP d<75pc S/N>8 K4V N=52

<V-Ic>(K4V) = 1.172 (+-0.012 sem, +-0.065 stdev; Gray01/03/06 N=46)
<V-Ic>(K3V) = 1.172 (+-0.025 sem, +-0.140 stdev; HIP d<75pc S/N>8 N=58)
(V-Ic)(K4V) = 1.20  ; TW PsA (Hipparcos)

(V-Ks)(K4V) =  2.441 ; exemplar HD 285742 (see below)
(V-Ks)(K4V) =  2.464 ; exemplar HD 285720  (see below)
(V-Ks)(K4V) = 2.649 ; trend for K dwarfs (standards+exemplars)
(V-Ks)(K4V) = 2.65  ; sec. stan. GJ 879/TW PsA (see below)
(V-Ks)(K4V) =  2.655 ; exemplar GJ 775 (see below)
(V-Ks)(K4V) =  2.697 ; exemplar HD 105671 (see below)
(V-Ks)(K4V) = 2.702 ; pri. stan. GJ 689 (see below)
(V-Ks)(K4V) = 2.703 ; fit for B-V=1.090
(V-Ks)(K4V) =  2.704 ; exemplar HD 24916 (see below)
=> adopt (V-Ks)(K4V) = 2.70 [updated 12/26/2020] 

<V-J>(K4V)   = 2.048 (+-0.027 sem, +-0.195 stdev; HIP d<75pc S/N>8 N=58)
<V-H>(K4V)   = 2.596 (+-0.028 sem, +-0.235 stdev; HIP d<75pc S/N>8 N=58)
<V-Ks>(K4V)  = 2.735 (+-0.033 sem, +-0.250 stdev; HIP d<75pc S/N>8 N=58)
<J-H>(K4V)   = 0.542 (+-0.009 sem, +-0.054 stdev; HIP d<75pc S/N>8 N=58)
<H-K>(K4V)   = 0.145 (+-0.007 sem, +-0.036 stdev; HIP d<75pc S/N>8 N=58)
<Bt-Vt>(K4V) = 1.316 (+-0.015 sem, +-0.109 stdev; HIP d<75pc S/N>8 N=58)
<V-Vt>(K4V)  =-0.130 (+-0.003 sem, +-0.032 stdev; HIP d<75pc S/N>8 N=57) 
<B-K>(K4V)   = 3.846 (+-0.043 sem, +-0.317 stdev; HIP d<75pc S/N>8 N=58)

(g-r)(K4V) = 0.921 ; Jordi06 (V-R)=>(g-r) for V-R=0.642
(g-r)(K4V) = 0.985 ; Bilir05 (B-V)=>(g-r) for B-V=1.10
(g-r)(K4V) = 1.00  ; Covey07
=> adopt (g-r)(K4V) = 0.99 [updated 10/10/2020]

Teff(K4V) =  4390 K ; exemplar 70 Oph B
Teff(K4V) = 4499 K ; pri. stan. GJ 689 
Teff(K4V) =   4500 K ; Bessell79
Teff(K4V) =   4507 +- 58 K ; Boyajian12b (N=1)
Teff(K4V) = 4529 K ; Ramirez05 Teff calibration for B-V=1.09
Teff(K4V) = 4542 K ; interp pri/sec/ter standards for [Fe/H]=0.0
Teff(K4V) =   4550 K ; GrayCorbally09 
Teff(K4V) =   4590 K ; Schmidt-Kaler82
Teff(K4V) = 4596 K ; EEM N=67 Teffs
Teff(K4V) = 4599 K ; sec. stan. GJ 879/TW PsA
Teff(K4V) = 4607 K ; Teff for d<30pc [Fe/H]=+-0.1 dex dwarfs from Hinkel17 for K4V for [Fe/H]=0.0
Teff(K4V) = 4615 K ; mean photometric Teff
Teff(K4V) = 4616 K ; ter. stan. GJ 570A (mean PASTEL Teff, see below)
Teff(K4V) =  4616 K ; exemplar HD 105671 
Teff(K4V) = 4622 K ; Teff for d<30pc [Fe/H]=+-0.1 dex dwarfs from Hinkel17 for B-V=1.10
Teff(K4V) = 4623 K ; [Fe/H] vs. Teff trend for standards & exemplars for [Fe/H]=0.0
Teff(K4V) = 4635 K ; Teff for d<30pc K4Vs in Hinkel17 for PASTEL -0.15<[Fe/H]<0.15 for [Fe/H]=0.0
Teff(K4V) = 4642 K ; smoothed Teff(SpT) trend
Teff(K4V) =  4642 K ; exemplar GJ 845 = eps Ind A
Teff(K4V) =  4650 K ; exemplar GJ 775
Teff(K4V) = 4651 K ; Casagrande10 Teff calibration for B-V=1.09
Teff(K4V) =   4690 K ; GrayCorbally94
Teff(K4V) =  4718 K ; exemplar HD 285742 (Hyad, metal-rich)
Teff(K4V) =  4762 K ; exemplar HD 285720 (Hyad, metal-rich)
=> adopt Teff(K4V) = 4600 K (logT = 3.663) [last updated 12/15/2020]

BCv(K4V) = -0.549  mag   ; EEM fit to Casagrande06/08/10 data (Teff=4600K)
BCv(K4V) = -0.542  mag   ; Masana06(Teff=4600K)
BCv(K4V) = -0.533  mag   ; Flower96(Teff=4600K)
BCv(K4V) = -0.520+-0.037 ; sec. stan. GJ 570A (Pecaut13)
BCv(K4V) = -0.513  mag   ; Bessell98(ATLAS9,logg=4.5,Teff=4600K)
BCv(K4V) = -0.502  mag   ; Casagrande18(Teff=4600K,logg=4.5)
BCv(K4V) =  -0.471+-0.018 ; exemplar GJ 845 = eps Ind (Pecaut13)
BCv(K4V) =  -0.465+-0.079 ; exemplar GJ 845 = eps Ind
BCv(K4V) = -0.437 mag    ; Bertone04(ATLAS,Teff=4600K)
BCv(K4V) =  -0.42  mag   ; Schmidt-Kaler82 (BCv = -0.55 + 0.13 offset)
BCv(K4V) = -0.331+-0.07  ; ter. stan. GJ 570A = HD 131977 (calc from Rains20)
=> adopt BCv(K4V) = -0.52 mag [last updated 12/15/2020; scaled so BCv(5772K)=-0.085]

Mv(K4V)  =  7.51 mag ; exemplar 70 Oph B = GJ 702B 
Mv(K4V)  =   7.4 mag ; GrayCorbally09
Mv(K4V)  = 7.19 mag ; pri. stan. GJ 689 = HD 160964 (~solar) 
Mv(K4V)  = 7.15 mag ; median SIMBAD K4V d<25pc (N=49)
Mv(K4V)  =  7.14 mag ; V-Ks=2.733 => EEM fit to Jao16 data
Mv(K4V)  =   7.12 mag ; V-Ks=2.733 => EEM fit to MS data
Mv(K4V)  =  7.10 mag ; V-K=2.733 => EEM fit to Reid CNS3 data 
Mv(K4V)  = 7.08 mag ; SIMBAD d<20pc trend for K dwarfs for V-Ks=2.70 (M_Ks=4.38)
Mv(K4V)  =  7.07 mag ; M_Ks=4.37 (median K4V d<25pc), V-Ks=2.79
Mv(K4V)  = 7.07 mag ; sec. stan. GJ 879 = HD 216803 (~solar)
Mv(K4V)  = 7.03 mag ; B-V=1.10 => EEM fit to SIMBAD dwarfs d<25pc
Mv(K4V)  = 7.025 mag ; trend [Fe/H] vs. Mv for standards & exemplars
Mv(K4V)  = 7.02 mag  ; Koen10 d<25pc median K4V
Mv(K4V)  = 7.02  mag ; median Mv K4V d<20pc XHIP
Mv(K4V)  = 7.01 mag ; CATSUP -0.1<[Fe/H]<0.1 d<30pc for B-V=1.10
Mv(K4V)  =   7.0 mag ; Schmidt-Kaler82
Mv(K4V)  = 6.985 mag ; xHIP d<25pc -0.1<[Fe/H]<0.1 for B-V=0.99
Mv(K4V)  = 6.98 mag ; B-V=1.10 => Mv(Wright05)
Mv(K4V)  =  6.98 mag ; exemplar HD 105671 = GJ 9391A 
Mv(K4V)  =  6.93 mag ; exemplar GJ 775 = HD 190007 ([Fe/H]~+0.16)
Mv(K4V)  =  6.89 mag ; exemplar GJ 845 = eps Ind ([Fe/H]~-0.13)
Mv(K4V)  = 6.88 mag ; fit to Yee17 Teff/Rad/logL data for K dwarfs for [Fe/H]=0, Teff=4620K
Mv(K4V)  = 6.87 mag ; ter. stan. GJ 570A = HD 131977 
Mv(K4V)  =   6.86 mag ; Wegner07
Mv(K4V)  =  6.64 mag ; exemplar HD 285720 = BD+17_715 (Hyad)
Mv(K4V)  =  6.58 mag ; exemplar HD 285742 = V991 Tau (Hyad)
=> adopt Mv(K4V)   = 6.98  mag [updated 12/15/2020]
=> adopt M_G(K4V)  = 6.56  mag [updated 12/15/2020] (Mv=6.98, G-V=-0.42)
=> adopt M_Ks(K4V) = 4.247 mag [updated 12/15/2020] (Mv=6.98, V-Ks=2.733)

(EEM note 12/15/2020: the [Fe/H] vs. Mv trend for the standards &
exemplars yields: Mv = 7.025 - 1.478*[Fe/H], rms=0.247 mag; the
standards are all within +-0.2 dex of solar, with several close to
solar. Using xHIP for K dwarfs d<25pc within +-0.1 dex of solar, the
trend for B-V=1.09 => Mv=6.985)

M_G(K4V) = 6.5148   HD_131977_  ; K4V_
M_G(K4V) = 6.68736  V*_TW_PsA_  ; K4Ve_
M_G(K4V) = 6.99891  *_70_Oph_B_ ; K4V_

M_Ks(K4V) = 4.37 ; median K4V d<25pc N=36
M_Ks(K4V) = 4.38 ; SIMBAD d<20pc trend for K dwarfs for V-Ks=2.70
M_Ks(K4V) = 4.42 ; sec. stan. GJ 879 = HD 216803 = TW PsA
M_Ks(K4V) = 4.49 ; pri. stan. GJ 689 = HD 160964 

logL(K4V) = -0.365+-0.068 ; ecl. binary ASAS_J045304-0700.4B (Helminiak11) 
logL(K4V) = -0.500+-0.052 ; ecl. binary EPIC 219511354B in Ruprecht147 [Fe/H]=+0.07+-0.03 (Torres21)
logL(K4V) = -0.68  ; exemplar GJ 845 = eps Ind
logL(K4V) = -0.688 ; Mv=6.98, BCv=-0.52 => Mbol = 6.460
logL(K4V) = -0.72  ; ter. stan. GJ 570A = HD 131977
=> adopt logL(K4V) = -0.688 dex [updated 12/28/2020]
=> adopt Mbol(K4V) =  6.460 mag [updated 12/28/2020]

Mass(K4V) = 0.822+-0.016 Msun ; EPIC 219511354B in Ruprecht147 [Fe/H]=+0.07+-0.03 (Torres21)
Mass(K4V) = 0.828+-0.0040 Msun ; ecl. binary ASAS_J045304-0700.4B (Helminiak11) 
Mass(K4V) = 0.746 Msun ; EEM Mv vs. Mass fit to Torres10 binaries (Mv=7.00)
Mass(K4V) = 0.755 Msun ; Eker18 calibration for logL=-0.708
Mass(K4V) = 0.748 Msun ; Mann18 calib for M_Ks=4.147
Mass(K4V) = 0.728 Msun ; EEM logL vs. Mass fit for binaries (logL=-0.740)
Mass(K4V) = 0.722 Msun ; Eker15 calibration for logL=-0.708
=> adopt Mass(K4V) = 0.75 Msun [updated 7/20/2020]

Rad(K4V) =  0.851+-0.016   Rsun ; ecl. binary EPIC 219511354B in Ruprecht147 [Fe/H]=+0.07+-0.03 (Torres21)
Rad(K4V) =  0.833+-0.005   Rsun ; ecl. binary ASAS_J045304-0700.4B (Helminiak11) 
Rad(K4V) =  0.790+-0.053   Rsun ; exemplar HD 285742 (Stassun19)
Rad(K4V) =  0.776+-0.061   Rsun ; exemplar HD 285720 (Stassun19)
Rad(K4V) =  0.7390+-0.0190 Rsun ; Boyajian12b (N=1)
Rad(K4V) =  0.738+-0.062   Rsun ; exemplar GJ 845 (Stassun19)
Rad(K4V) =  0.726+-0.050   Rsun ; exemplar GJ 9391A (Stassun19)
Rad(K4V) = 0.717+-0.061   Rsun ; ter. stan. GJ 570A (Stassun19)
Rad(K4V) = 0.715+-0.009   Rsun ; ter. stan. GJ 570A = HD 131977 (Rains20)
Rad(K4V) = 0.7131         Rsun ; Teff=4600K, logL=-0.688
Rad(K4V) =  0.711+-0.005   Rsun ; exemplar GJ 845 = eps Ind ([Fe/H]=-0.13; Rains20)
Rad(K4V) =  0.710+-0.058   Rsun ; exemplar GJ 702B  (Stassun19)
Rad(K4V) =  0.691+-0.056   Rsun ; exemplar GJ 775 (Stassun19) ([Fe/H]~+0.16) 
Rad(K4V) =  0.670+-0.009   Rsun ; exemplar 70 Oph B = GJ 702B (Yee17)
Rad(K4V) = 0.696+-0.054   Rsun ; pri. stan. GJ 689 (Stassun19)
Rad(K4V) = 0.692+-0.046   Rsun ; sec. stan. GJ 879 (Stassun19)
=> adopt Rad(K4V) = 0.713 Rsun [updated 12/28/2020] 

# No standard

JM53, Wilson62, Morgan65, MK73, Houk75, KeenanMcNeil76, Corbally86,
and Garrison94 do not list a K4V standard.

# Primary Standard

GJ 689 = HD 160964 
 *K4V: Keenan80,Keenan83,Keenan85,Keenan88,Keenan89
  K7V: Reid04
  K8V: Vyssotsky52,Woolley70,Gliese91

  Unfortunately a few discrepent published types, however Keenan
  consistently listed it the K4V standard throughout the 1980s (and it
  is approximately solar metallicity).  V=8.620(Mermilliod91,Hauck90),
  V=8.55(HIP), V=8.554+-0.014(TYC2=>Johnson), V=8.59(Tycho),
  V=8.603+-0.018(Droege07) => adopt V=8.57. B-V=1.100(Mermilliod91),
  B-V=1.089+-0.011(HIP), V-I=1.16+-0.02(HIP), J=6.524+-0.021(2MASS),
  H=6.011+-0.021(2MASS), Ks=5.888+-0.020(2MASS), V-Ks = 8.59-5.888 =
  2.702. Only PASTEL Teff: 4499+-8K(Mishenina08,Mishenina12),
  [Fe/H]=-0.07+-0.05(Mishenina12), [Fe/H]=0.04(Luck17) => adopt [Fe/H]
  = -0.015, i.e. ~solar, <logg>=4.65(N=3).  plx =
  52.4550+-0.0261mas(GaiaDR2), plx=52.4929+-0.0167mas(GaiaEDR3). Mv =
  7.19, M_Ks=4.49.  Gaia DR2: G=8.1642+-0.0004, E(BR/RP)=1.237, Bp-Rp=1.3493,
  Bp-G=0.6293, G-Rp=0.7200 => G-V = 8.1642 - 8.59 = -0.426. GaiaEDR3:
  G=8.170528+-0.002764, E(BP/RP)=1.248, Bp-Rp=1.337254, Bp-G=0.599900,
  G-Rp=0.737354 => G-V = 8.170528 - 8.57 = -0.3995.

# Secondary Standard

GJ 879 = HD 216803 = TW PsA = Fomalhaut B = HR 8721 (J2256-3133)
  K3V: Popper47,Wilson62,Bidelman85(K3)
 *K4V: Keenan85(standard),Keenan89(K4Ve,non-stan),Houk3(K4Vp),Pecaut13
  K4+Vk: Gray06
  K5V: Joy47,Evans57,Upgren72

  It various from K3-K5 in the literature. One of only two K4Vs in the
  BSC. Companion to Fomalhaut, and only ~440 Myr
  (Mamajek12). Jaschek64 lists one K4V ref, and 3 K5V refs! In
  Keenan89, but not listed as a standard.  V=6.48(HIP),
  B-V=1.094(HIP), V-I=1.20(HIP), K=3.83(Aumann91), 2MASS H & K
  photometry saturated. (V-Ks)=2.65(HIP,Aumann91), probably within
  +-0.03 mag of 2MASS system. A few PASTEL compendium spectroscopic
  parameters: Teff = 4599+-27K(N=3), [Fe/H]=0.03+-0.04(N=2),
  logg=4.71+-0.18(N=2).  plx = 131.4380+-0.0856mas(GaiaDR2). Mv =
  7.07, M_Ks=4.42.

# Tertiary Standard

GJ 570A = HD 131977 = HR 5568 = StKM 2-1093 = KX Lib
 *K4V: Cowley67,Abt81,Bidelman85,Stephenson86,Keenan89(non-stan),GrayNstars(only K4V stan),Houk4(blended),Gray06
  K5V: Adams35,1942HarCi.448....1H,Buscombe59,Evans66(K5Ve),Reid04

  The other K4V in the BSC. Since 1967 8 K4V classifications, and one
  as K5V. Nearby and well-studied and interferometrically measured
  diameter (Rains20). V=5.72(HIP), V=5.71(Johnson66), B-V=1.024(HIP),
  B-V=1.11(Johnson66), U-B=1.06(Johnson66), V-I=1.23+-0.08(HIP), 2MASS
  photometry is saturated. plx=170.0112+-0.0851mas(GaiaDR2).
  Mv=6.87. Many estimates in PASTEL (N=17), mean properties Teff =
  4616+-34K, [Fe/H] = +0.02+-0.04, logg=4.31+-0.14.  But B-V is a bit
  bluer than <B-V> for K4Vs. Companion is M1.5V standard star ~25"
  away, but has been as close as 9.4" in past. Quoted photometry might
  be blended?  Washington Double Star catalog lists V mags of 5.88 for
  A and 8.18 for B. Rains20: tetLD=1.130+-0.014mas,
  Rad=0.715+-0.009Rsun, fbol = (17.6+-1.1)e-8 erg/s/cm2 (=>
  mbol=5.389+-0.068 IAU2015 scale), Teff=4505+-76K, L=0.19+-0.01Lsun
  (=> logL = -0.72+-0.02). BCv = mbol - V = 5.389+-0.068 - 5.72 =
  -0.331+-0.07.

# Deprecated Standards

GJ 719 = HD 234677 = BY Dra 
 *K4Ve+K7.5Ve : Keenan80(K4V+K7.V),Keenan89(SB!)
  K4Vke : Gray03
  K5: Bidelman85(K5)
  K6Ve : Gliese91
  K7V: Stephenson86
  dM0e : Pourbaix05(Va=8.04,Vb=8.48!)

  Skiff07 shows range in types from
  K5V-M1V. Keenan80(1980PASP...92..548K, not the catalog) describes
  the composite spectrum "shortward of about 4300A the type is K4V,
  while from the blue-green to the red a type of about K7.5 fits".
  Conclusion: poor choice of standard. Keenan80 estimates the
  individual magnitudes to be V(A)=8.66, V(B)=9.62.

# Exemplars

GJ 845 = eps Ind = HR 8387 = HD 209100 = LHS 67 = HIP 108870 
   K3V: Woods55,Upgren72
  *K4V: Moore32(K4),Gray06(K4V(k))
   K5V: 1923BHarO.789....2S,Adams35,Hoffleit37,Evans57(K5Ve),Houk75(K4/5V=>K5V following Pecaut16)

   Adopting Gray06's modern classification. Although the published
   types range from ~K3-K5, its B-V and Teff compare well to the K4V
   standards. Nearby and has interferometrically measured diameter, so
   is well characterized.  V=4.688+-0.012(Mermilliod91),
   B-V=1.060+-0.007(Mermilliod91),
   U-B=0.993+-0.005(Mermilliod91). PASTEL median values 3/22/2018:
   <Teff> = 4642K (N=15), <[Fe/H]> = -0.13 (N=7).  Best: Teff =
   4649+-36K, [Fe/H]=-0.19+-0.07, logg=4.63+-0.01 (Ramirez13).  2MASS
   photometry is blasted (DCD).  Binary T dwarf companions 402" away,
   and Feng18 claims Jupiter-like planet orbiting A.  Rains20 reports:
   tetLD=1.817+-0.013mas, Rad=0.711+-0.005Rsun, fbol=(51.5+-3.7)e-8
   erg/s/cm2 (mbol = 4.223+-0.078), Teff=4649+-84K, L=0.21+-0.02 =>
   logL = -0.68+-0.04. BCv = mbol - V = 4.223+-0.078 - 4.688+-0.012 =
   -0.465+-0.079. plx = 274.8048+=0.2494mas(GaiaDR2),
   276.06+-0.28mas(HIP2).  Mv = 6.893+-0.012.

GJ 775 = HD 190007 = BD+02_4076 = V1654 Aql 
   K2: Cannon
   K3/4V: Houk99(following Pecaut16 => K4V)
  *K4V: Joy49,Jacshek78(Wilson62),Gray03,Gray06(K4V(k))
   K5V: Kirkpatrick91(sec)

   Kirkpatrick91 uses it as a K5V standard, but others consistently
   classify it hotter. Gray03 (using 61 Cyg A [K5V] and 61 Cyg B [K7V]
   as standards) classifies it one subtype hotter. Its color is not
   that different from the other K5V standards,
   however. V=7.451+-0.011(Mermilliod91),
   B-V=1.141+-0.011(Mermilliod91), U-B=1.081+-0.014(Mermilliod91),
   J=5.476+-0.024(2MASS), H=4.956+-0.026(2MASS),
   Ks=4.796+-0.017(2MASS).  V-Ks = 2.655 (Mermilliod91,2MASS).  PASTEL
   median values: <Teff> = 4650K (N=6), logg=4.38(N=4),
   [Fe/H]=0.155(N=4). plx=78.6238+-0.0617mas(GaiaDR2). Mv=6.93. 

GJ 9391A = HD 105671 = GJ 456.1A = HIP 59296 
  *K4V: Houk78,Lu83,Gray06
   K5V: Evans64(Jaschek78),Bidelman85(K5)

   V=8.46(HIP), B-V=1.137+-0.003(HIP), J = 6.441+-0.018(2MASS), H =
   5.913+-0.036(2MASS), Ks= 5.763+-0.021(2MASS), V-Ks =
   2.697(HIP,2MASS). PASTEL compendium: <Teff> = 4616K(N=5),
   logg=4.415(N=2), [Fe/H]=-0.02(N=3; large scatter).
   plx=50.6116+-0.0488mas(GaiaDR2). Mv=6.98.

GJ 702B = 70 Oph B = HD 165341 B
   K3V: Greenstein52
  *K4V: Morgan38,Slettebak63,Cowley67
   K5:  Bidelman85
   K6V: Adams35

   B-V=1.15(Gliese91), V=6.00(Gliese91),
   V=6.061+-0.021(Eggenberger08).  Poor 2MASS
   photometry. [Fe/H]=0.04+-0.05(Eggenberger08). Teff=4393+-149K(Boyajian12),
   4390+-200K(Eggenberger08) => adopt Teff=4390K, although this seems
   ~-200K from other K4Vs of similar
   metallicity. M=0.73+-0.01Msun(Eggenberger08).  plx=195.2166+-0.1012
   mas(GaiaDR2). Mv=7.51+-0.02.  Boyajian12 reports
   tetLD=1.221+-0.015mas => w/GaiaDR2
   parallax=>Rad=0.672Rsun. <[Fe/H]>=-0.02(Gaspar16)

HD 285720 = BD+17_715 = 2MASS J04241691+1800107 (Hyad)
   K2: Nesterov95
  *K4V: Wilson48(K4),Morgan65
   K6V: Joy49

   One of two Hyads classified K4V by Morgan65.
   V=9.988+-0.011(Mermilliod91), B-V=1.050+-0.011(Mermilliod91),
   U-B=0.950+-0.000(Mermilliod91), H=8.154+-0.018(2MASS),
   H=7.636+-0.023(2MASS), Ks=7.524+-0.018(2MASS), V-Ks = 9.988 - 7.524
   = 2.464. plx = 21.4034 +- 0.0428 mas (Gaia DR2). Mv =
   6.640+-0.012(Mermilliod91,GaiaDR2).  Teffs:
   4712K(Casagrande11),4750K(GaiaDR2),4774K(GaiaDR2#1),4894K(Huber17)
   => <Teff> = 4762K.

HD 285742 = V991 Tau = 2MASS J04250024+1659057 (Hyad)
   K2: VanRhijn34,Wilson48
  *K4V: Ramberg41,Joy49,Morgan65

   One of two Hyads classified K4V by Morgan65.
   V=10.271+-0.012(Mermilliod91), B-V=1.030+-0.004(Mermilliod91),
   U-B=0.929+-0.012(Mermilliod91), J=8.486+-0.023(2MASS),
   H=7.980+-0.024(2MASS), Ks=7.830+-0.016(2MASS), V-Ks = 10.271 -
   7.830 = 2.441. plx = 18.2821 +- 0.0622 mas (GaiaDR2). Mv =
   6.581+-0.014(Mermilliod91,2MASS).  G = 9.9678+-0.0005, Bp-Rp =
   1.2361, Bp-G = 0.5594, G-Rp = 0.6767. Teffs: 4678K(Luo18#1),
   4704K(Luo18#2), 4718K(Casagrande11), 4798K(GaiaDR2), 5000K(GaiaDR2)
   => <Teff> = 4718K.
 
# Other stars (not standards)

HD 24916 = GJ 157 A = HIP 18512 = LTT 1830 = RE J0357-011 = UGP 71 =
   K3: Luo15(LAMOST)
  *K4V: Leonard44,Wilson62,Bidelman85(K4),Stephenson86,Gray03
   K4+Ve: Popper43
   K5V: Joy47,Upgren72,Luo16(LAMOST),Luo18(LAMOST)

   Has M2.5V CPM companion 11" away. V = 8.045+-0.005(Mermilliod91),
   B-V = 1.115+-0.005(Mermilliod91), U-B=1.015+-0.015(Mermilliod91).
   J=6.063+-0.023(2MASS), H=5.486+-0.026(2MASS),
   Ks=5.341+-0.018(2MASS).  V-Ks = 2.704 (Mermilliod91,2MASS).  T
   LAMOST reports spectroscopic parameters 4 times, with the most
   recent (Luo18): Teff = 4433+-37K, logg=4.666+-0.054, [Fe/H] =
   -0.108+-0.030.

GJ 141 = HD 21197:
   K4V: Wilson62,Houk99(Pecaut16=>K4+V),Gray03
   K5V: Roman55,Stephenson86,Upgren72,Gliese91
   K6V: Adams35

   Probably intermediate ~K4.5V

HD 154712 (V=8.6)
   K3V: Adams35
  *K4V: Roman55
   K5: Bidelman85

   CPM companion 12" away with K4 type (Bidelman85)


### TRENDS FOR K4V STARS

xHIP catalog d<30pc, K4V stars with classifications post-2000 (mostly Gray03/Gray06), with [Fe/H] values

 HIP     Plx   SpType   [Fe/H] eFeH    B-V   eB-V   rSpType   
 75542   41.50  K4V     -0.31  0.03   1.045  0.017  2006AJ....132..161G      
  4454   40.90  K4V     -0.20  0.06   1.025  0.024  2003AJ....126.2048G      
 33955   54.22  K4V     -0.13  0.03   1.078  0.008  2003AJ....126.2048G      
 54952   68.28  K4V     -0.05  0.02   1.043  0.005  2008ApJS..176..216A      
113283  131.42  K4V     -0.01  9.99   1.094  0.006  2006A&A...460..695T                     
 25220   71.00  K4V      0.02  0.02   1.104  0.001  2003AJ....126.2048G      
 73184  171.22  K4V      0.02  9.99   1.024  0.015  2006AJ....132..161G      
103256   44.61  K4V      0.02  0.06   1.054  0.005  2003AJ....126.2048G      
 62229   48.79  K4V      0.05  0.04   0.937  0.002  2006A&A...460..695T      
 59296   50.59  K4V(k)   0.10  0.02   1.137  0.003  2006AJ....132..161G      
 21818   75.82  K4Vke    0.11  9.99   1.092  0.005  2003AJ....126.2048G      
 98698   77.76  K4V(k)   0.12  9.99   1.128  0.015  2006AJ....132..161G      
  5286   47.50  K4V      0.14  0.03   1.122  0.012  2003AJ....126.2048G      
 47690   54.89  K4V      0.20  9.99   1.133  0.007  2003AJ....126.2048G      
 15919   64.97  K4V      0.23  0.02   1.153  0.010  2003AJ....126.2048G

for those with [Fe/H] uncertainties:
B-V = 1.084 + 0.2773*[Fe/H]  (rms = 0.023 mag)

higher order fits don't help. Support for (B-V) = 1.084+-0.02