Hi NoTape,
In /boot/config.txt
Change
dtoverlay=vc4-kms-v3d-pi4,cma-384
To
dtoverlay=vc4-fkms-v3d-pi4,cma-384
Or
dtoverlay=vc4-fkms-v3d,cma-384
Now run
cat /proc/device-tree/soc/firmwarekms@7e600000/status
You should see
MLD> cat /proc/device-tree/soc/firmwarekms@7e600000/status
okayMLD>
After that try to play some 4K contents and see if it works.
Best Regards.