HT3526: Adding additional media format support to QuickTimeLearn about Adding additional media format support to QuickTime
Currently Being ModeratedJul 11, 2013 11:48 AM (in response to ignoramouse)
Quicktime should be able to play mp4
Perhaps like it says, there are parts of the file that have other codecs used, so to play it back, use the free VLC player instead.
Direct link is here, don't install from other sources as they try to install adware or malware.