cvt 3840 2160 13.8
# 3840x2160 13.79 Hz (CVT) hsync: 30.05 kHz; pclk: 144.25 MHz
Modeline "3840x2160_13.80" 144.25 3840 3944 4320 4800 2160 2163 2168 2180 -hsync +vsync

xrandr --newmode "3840x2160_13.80" 144.25 3840 3944 4320 4800 2160 2163 2168 2180 -hsync +vsync
xrandr --addmode HDMI1 "3840x2160_13.80"

# if unknown
xrandr --output HDMI1 --mode 3840x2160_13.80