Yeah, you're right. Depends on what's in the AVI, of course. If the input is already something that can be housed inside mp4, i.e. standard codecs, the packets can survive by just copying them.
Sure, you can mux DivX into a mp4 container, but don't be surprised if it wont play in various equipment that don't support that specific codec. H264 should be working mostly everywhere, I hope :-)