If the MPEG-4 video was encoded with the Advanced profile, it may not play back on some devices. Therefore to avoid playback issues make sure that when creating or selecting a .MP4 video that it is encoded using a Simple Profile.
The suffix .MP4 on a video file represents a MPEG-4 file. MP4 is simply a container used to package video and audio streams. There are still a wide range of encoding profiles that can be used which are selected during the video encoding or conversion process.
Although the videos are in the correct format the encoding may be incorrect and therefore will prevent playback. MP4 files can be encoded in either the Simple or Advanced profile.
NOTE: For additional information regarding MPEG-4 encoding or any other information in this solution, refer to the help guide supplied with the software used to create or attain the .MP4 file.