you might need to play with the dtoverlay= kms or fkms values in config.txt. i know the vc4 driver isnt fully ready to drive the vc5 hardware in the pi4 but i know that for example my omxplayer-pi snap works fine (only 1080p though, omxplayer does not support h265 yet) when either using no dtoverlay or with the fkms one (not with the kms one though).
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