Think of MP4 as a mere container. It can contain different kinds of video codecs.
Mac OS’s built in QuickTime Player does not support all codecs (for example H.265).
Apple likes to promote their own video codecs, so they don’t care about implementing playback for others.
Try the video player VLC Player. It is free, open source and plays pretty much anything.
In the info-window you can check what video and audio codec your MP4-file actually uses.
To transcode to a different video format i recommend the free open source app Handbrake.
Hope this helps, cheers!
PS: you might already know, but these compressed formats are only worth for previews and easy demonstration. They are not usable in a professional production pipeline. In that case rather use Apple ProRes, Avid codecs or sequences of uncompressed images (best)