Check your digital mp4 player manual and find the supported video specs (resolution, encoder, format, bit rate, frame rate). Then, convert your video with video converter.
Super is nice for PC user.
iSkysoft is nice for Macintosh user.
First of all, we kindly advise you to make a confirmation that what video format your mp4 player support.
If your movie format is not compatible with MP4 player, you should use an video converter program to convert it.
Free video conveter--http://www.avcware.com/downloads/avc-video-converter-free.exe