Hi Rontopia,
I'm not entirely sure of the full history but have been told by manufacturers of these players that the main distinction they put between "mp3" and "mp4" is that "mp3" players only play music and NO video, while "mp4" players can also play music and VIDEO.
The MP4 format itself won't play on these players straight up, but needs to be converted. Its like saying it can play "avi" and "wmv" videos, however NEEDS TO BE CONVERTED firstly.
Hope that helps.
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