This VideoHelp page lists a few freeware utilities that support the conversion of MP4 to AVI, such as the following:
If you are just looking for conversion to DivX, the "Dr. DivX" will probably do the trick as this tool mentions it supports MP4 as an input.
Another one worth trying is the utility 'SUPER'. Despite its huge file package (since its bundled with various codecs), it seems to handle pretty much every common video codec, including the MPEG4 AVC codec and transport streams. However, one thing to watch out for is that a few user reviews mentioned about this causing WMV playback issues after installation, although this hasn't happened with me.