Thanks for your replies! If I go to display settings the other monitor doesn't show up:
It looks like it does see the monitor here (DP-4):
$ xrandr --verbose
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 32767 x 32767
HDMI-0 connected primary 1920x1080+0+0 (0x1bd) normal (normal left...