@RAOF has been testing Mir on RPi4 https://github.com/MirServer/mir/issues/1663 we’re now getting pixels lit on screen, which is an improvement, but there are still issues to resolve.
The relevant bugs are:
The first one has a Bionic task, and that’s where you can look for whether the fix will be backported to 18.04.
To be clear: this is testing Mir on RPi4 on Ubuntu Core 18; you shouldn’t need to switch to core 20 images.
Edit: Whoops, no, sorry. This was a self-built Core 20 image.