The correct way of doing things would be to create a new plugin / player combo for MPV, just like phenigma did for the rpi with omx plugin and omx player. It is not wizardry, and a lot of existing code can be copy and pasted, but it is a road that will take a few month to complete.
OR, use ffmpeg to recode the mp4 so that the regular players can play them. It is my experience, that you will always find files that someone encoded in a way that some players can't play them as good as others. The engine that is best at playing files is a round-robin. This week it might be MPV, next week it is the latest incarnation of VLC, followed by some gstreamer based system two weeks from now.