Moviemaker doesn't seem to do MP4
It does do MP4 - I use it for knocking together clips from my Nokia.
There are a few types of MP4 I think - I had issues getting mine working, but I downloaded some codecs and it sorted the problem. Unfortunately I can't remember what codecs or where from. But if you can get it working, it's the easiest and quickest program by far (alongside imovie), and perfect for bashing together low quality stuff.