Something went wrong with my last comment (should've been posted yesterday, and this morning I got some error), but anyway.
Yeah, I could check a 'local mp4' box in the settings of the 'Simple Video Audio Converter' package for SMF forums, so that links to mp4 files can be played like YouTube links. I've also added mp4 to allowed attachments, but these don't seem to play automatically. I'll see if I can fix that, but even if I can, I fear that mp4 attachments may take up too much space on the server. I don't know what the data limits/costs are.