Though QuickTime Player is not that satisfying to play MP4 files on Mac, you can still use it to play MP4 videos of any kind with the help of a video converter. Plus, there are also better Mac MP4 player alternatives to QuickTime to let you watch MP4 videos movies on Mac without issues. Find them out here.
VLC media player, as the most popular and powerful media player for Mac, Windows, iOS, Android, etc. can be the best QuickTime alternative to play MP4, MOV, MKV, AVI, WMV, etc. videos on Mac successfully. No matter MP4 clips, MP4 movies or online MP4 streams, VLC is cable of handling them. BTW, it is totally free and open-source.