dots/etc/xorg.conf.d/10-monitor.conf

Section "Screen"
  Identifier	"eDP1"
  Option 	"Primary" "true"
Endsection

Section "Monitor"
  Identifier 	"HDMI1"
  Option 	"PreferredMode" "3840x2160"
  Option	"LeftOf" "eDP1"
Endsection