Do you have FFDShow installed? MP4 is just an container, so you also need a program (like FFDShow) to decode actual video format. Most likely MP4 is in H.264 format.
This is the latest stable version of FFDShow:
http://www.afterdawn.com/software/vi..._610__20061201
After installing, open ffdshow's video decoder preferences (start-all programs-ffdshow-video decoder configuration) and go to the 'Codecs', then set H.264 to "libavcodec".