MP4 is just a container - the video and audio stream inside it will both be encoded in different formats.
Android natively only supports certain types of formats. There is a list here:
http://developer.android.com/guide/appendix/media-formats.html
Make sure the video and audio encoding type is...
@ShadowWizard well...it is a usual saying here. When someone takes responsibilities, they say that their(the one who now has lot of responsibilities) days of playing are over now.
of-course, the saying can be used in other circumstances too