@jwcph
My first thought is that a lot depends on the user's device, and the user's technical knowledge.
1) What is their default MP4 player? Do they have one set?
2) Do they know to save it to their device first, or are they clicking/tapping the link and treating it like streaming media?
3) If they're treating it like streaming media, are they playing the file in a location with good reliable bandwidth?
4) If they're playing it in-app, does the in-app media player support files of that size, or just little meme-sized video shorts?
You get the idea. Multiple symptoms probably indicate multiple causes.