So RetroPie/EmuStation runs at whatever resolution is being suggested by the hdmi connection?
As its working fine at HD when I start the desktop instead.
Does emustation use a different graphics driver than the desktop?
EDIT: Ok - So its the actual raspberry pi board that is somehow at fault. Iv'e crosschecked the sd card with another pi4 I had and with the suspected monitor and it's definitely the first rpi.