The problem is that my secondary monitor (4K) shows 1920x1080p as the maximum resolution when my primary monitor (1080p) is on. I tried to specify the monitor in the RetroArch settings, but still no good.
When only the 4K monitor/TV is on, it shows the 4K resolution.
Can anyone help me? I would like to be able to play while still using my 1080p screen to watch YouTube.