Hi NoTape,
In /boot/config.txt
Change 
dtoverlay=vc4-kms-v3d-pi4,cma-384To
dtoverlay=vc4-fkms-v3d-pi4,cma-384Or
dtoverlay=vc4-fkms-v3d,cma-384Now run 
cat /proc/device-tree/soc/firmwarekms@7e600000/statusYou 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.