QuickTime Player can't open "file.mp4"

My husband completed a movie in iMovie 10.0.5. When he tries to open said movie in QuickTime Player 10.3 he gets a window that says,

QuickTime Player can open FlyIn.mp4 (FlyIn is the name of the movie, obviously). We tried QuickTime Player 7.6.6 found in Utilities

and the mp4 is not even recognized so we can attempt to open. He's running Mavericks.😕