The extension "mp4" is not specific enough. All 'mp4' normally
indicates is the container format used to package together the
video/audio/subtitle/other data into a single file.
In order to play the file you would need need the proper demuxer to
extract the components from the container, and you need the proper
codec's to decode whatever compression format was used to compress the
video (and appropriate audio codecs for the audio channels) to be able
to play the file.
I'd guess that one or more of the above were missing.