I'm assuming you have the DivX codec installed to play it?
I think the best approach would be to first put it in a Quicktime container. This can be done via DivX Doctor or QTAmateur (saving just as a reference file). You then open the new .mov file in MetaHoot and change the video fourcc to read mp4v. After that you should be able to export to MP4 with video passthrough while encoding the audio to AAC.
Though I should point out that there's a risk that it won't play if the video doesn't match with the MPEG4 Simple Profile.