Freeplay CM4

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If I had to guess, the Pi4 image maybe uses
  dtoverlay=vc4-kms-v3d
and I think that does not use dispmanx which the fbcp relies on.  If you have that dtoverlay line in the config.txt, you may want to comment it out and use this instead
  dtoverlay=vc4-fkms-v3d

In fact, I think you may want to try to force a different HDMI resolution for testing purposes.

Try adding these lines to that Pi4 image's config.txt file (in addition to changing vc4-kms-v3d to vc4-fkms-v3d).


hdmi_group=2
hdmi_mode=87
hdmi_cvt=320 240 60 1 0 0 0
hdmi_force_hotplug=1


I would guess that MIGHT help the built-in LCD, but your HDMI will likely fail after that.  If so, try some different values for hdmi_cvt like
hdmi_cvt=640 480 60 1 0 0 0
hdmi_cvt=1024 768 60 1 0 0 0

OR switch to what we would use in the Zero/CM3 image.

Code:

framebuffer_width=320
framebuffer_height=240
hdmi_force_hotplug=1  #these HDMI lines will try to set up 1024x768 (you can remove them if you have problems playing via HDMI)
HDMI_FORCE_MODE=1
hdmi_group=2
hdmi_mode=16
hdmi_drive=1          #Normal DVI mode (No sound) (2 for HDMI with sound)