Hi Art,
I spent quite a long time categorizing practically every porn site for the WhoPaidMe project, and I have to say, there isn't all that many mobile sites out there just yet.
Here are some from the adult mobile niche that appear to support MP4 according to their websites...
handheldpornsites.com from
Hand Held Porn Sites
mobilexxxstore.com from
Adult Valley Cash
ipodstrip.com from
Rev Share Cash
ipocketvideos.com from
Adult Innovators
Here's a link to all of the
adult mobile sites that I placed in that niche. You can probably find a few more that support MP4.
Give me a shout if you need any more help.
Mike