MP4 can contain many different types of video, for mobiles right up to HDTV (and beyond), with various audio codecs, bitrates, resolutions, and even different forms of the mpeg4 codec (h263, h263, simple profile, high profile etc)
try using the above link, make sure they are the right format, and if you looked at the link you'd see that the software can also put the files on your iPod.

If you're so sure they don't need converting, drag them into iTunes, set the artist name, track name, and in the "more info" (right click) change it to the correct type (music video, TV show etc), plug in your iPod and let it sync.
If the video doesn't copy accross to the iPod then there's something wrong with the video (the iPod only accepts 2 types of MP4, up to a certain resolution and certain bitrate), just bcause it plays in iTunes doesn't mean it will work on the iPod.

I watch mp4 hi-def files all the time, some contain AAC 5.1 audio, some contain AC3 5.1 audio, some are x264 video codec, some are h.264 simple profile, some are h.264 high profile, all of which would not work in an iPod.

[Return iTool DVD to iPod for Mac]