Hello,
I faced a similar issue and found a solution in this post. I hope it proves helpful for you as well.

Hi zusammen, hat das Problem vll. schon jemand gelöst?
Habe das gleiche Problem das ich mein eigenes Video nicht drauf bekomme, das wird immer gelöscht.

lg

If your boot video isn't working, it may be due to incompatible file formats, corrupted files, or incorrect BIOS settings. Ensure the video is in a supported format, check your BIOS settings to enable boot videos, and verify that the video file is not corrupted. Updating your graphics drivers and BIOS firmware can also help resolve compatibility issues.

@nonameboy I don't have an Rpi4 so I can't test it, but there are no reports of other users with the same problem, and it doesn't make sense to work better on Rpi3 and not work on Rpi4.

What version of Recalbox are you using (say version, don't say "latest")?
Did you build your Recalbox yourself, or download a ready-made image from the internet?
Have you tried with another SD card?