.Xresources_4k

! This gives exactly 200% on my 4k monitor. Tested by rendering 1920x1080 window with a program that follows DPI settings — it filled the entire screen exactly.
Xft.dpi: 192

! These might also be useful depending on your monitor and personal preference:
! Xft.autohint: 0
! Xft.lcdfilter:  lcddefault
! Xft.hintstyle:  hintfull
! Xft.hinting: 1
! Xft.antialias: 1
! Xft.rgba: rgb

! nsxiv:
Nsxiv.bar.background: #000000
Nsxiv.bar.foreground: #FFFFFF