just a long shot, how did you copy your Mp4 files to your note? just drag and dropping them ?
Usually when your going to copy or move music/video files on to your Note, You will usually be prompted with a Window on your PC/laptop to say t"hat this file can be converted.. etc, and would you like to convert this file?", when you click on Convert... that will be the time that file/s will be converted to a readable file that the Note can read.. .and play...