AAC is supported (which is normally what goes in the mp4 (sometimes given the extension m4a) audio files) in its raw form in moonshell and some other players.
MP4 for video can be all manner of things but it is usually (anything else tends to appear in AVI, OGG, MKV or something even worse like quicktime/real that I have seen) AVC aka H264 video which is a bit beyond the DS. On the other hand someone (as I recall it was the person behind the DPG (amongst other things) encoder EZBuilder that done it) ported a version of xvid although nobody has picked up and ran with it yet (no audio for one). Search through the homebrew news or search gbadev if your are curious.