1. MP4 is a container. It could hold video, audio this TV can't decode and hang.
2. TVs are still years away from playing it all. For example, some think it plays on the PC so it should on the TV. (nope)
Try transcoding with HANDBRAKE or better yet, demand your supplier tests it on your TV first.
Whenever I want to watch a downloaded film or programme on my USB stick it freezes no matter what size the file is.
I normally make sure the format is Mp4 so can't see what the problem is. Once it freezes there is no way of getting past it......anyone else with this problem?