@LOGZ
In my humble opinion, there is a issue related to the videomode or even the memory, but I'm not getting the point.
I'm not a developer, but, as far as I understand, you're right, the PPSSPP usually has problems with the videomode and the lack of memory, but an RPi4 with 2gb of memory already has enough memory for that, so, the problem in this case it must be the videomode.
As output I use a Samsung TV Series6 4K by HDMI.
Rpi4 has a known bug in the detection of 4k TVs, it will be fixed in the next version:
https://gitlab.com/recalbox/recalbox/blob/master/CHANGELOG.md
Since your TV is 4k, I believe your problem is directly related to this.
I recommend you be patient and wait for the next version.