^ Yes, MC21 reads MP4 videos. I have tested with quite a few of them. In fact, MC21's CODEC handling is pretty exceptional. The only thing I've found that it doesn't like are some older MOV files. ...and a few oddball things here and there, like video 240p video taken with a very very old consumer digital camera.
Normal MP4 with H.264 video is no problem at all.
If you have a video file or two that it won't read, you should find out what codecs it uses internally.
Brian.