Note the .mp4 (or .m4v) extension on a file just indicates the container format. The video and audio data in MP4 files can be in a number of different codecs, some of which may not be supported by all players. For example, it's possible to have an MP4 file with the video encoded in AV1 or H.265/HEVC codecs, which are only likely to work on newer devices. Programs like MediaInfo will give more information about the codecs used for any given media file.
If it's older then it may only be intended to support older video formats like WMV (Windows Media) or MPEG4 Part 2 (variants include DivX and Xvid) which can be in an .mp4 file, but also .avi and others.
Or it could just be that it's only intended for music files as you mention...
Yes you can buy DVD or BluRay players which will read video formats from a USB stick. This is a Blu-Ray player for example that supports USB connection of sticks and hard drives and should read most common formats - https://www.richersounds.com/panasonic-dmpbdt180.html There are much cheaper DVD players however, I think it's unlikely they'll support all of the newer formats (the few I looked at claimed to be multi format players, but digging into the specs showed references to "DivX" video only).
Edited by jonuk76, 07 June 2024 - 04:40 AM.