There is very little software for editing MP4 type files, so far. It's a fairly new compression standrard. You're probably better off converting to DV-AVI and editing the results.
MP4 is a video compression method that sacrifices quality for smaller file size, much like the comression used for DVDs which is MP2 (or more properly called MPEG-2). Files so compressed are not really intended for further editing, outside of simple cut and paste operations... and even those can cause problems.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70