I don't know of a free MP4 editor. That doesn't mean there isn't a free MP4 editor - just that I don't know. I use SUPER to convert MP4 files to the AVI format that I can edit. Here's a link to the developer's website:
http://www.erightsoft.com/SUPER.html
It's fairly easy to use, and it's totally free. Here's the User's Guide in PDF format.
http://forum.videohelp.com/images/guides/p1608146/basicsuperguide.pdf
SUPER will convert just about any video file format to MOV, AVI, WMV, MPG, and a whole slew of other video file types. After conversion to the file type of your choice, you can edit it with whatever software you normally use.