I know this has been discontinued but does anyone know if it will play MP4s?  Like if the DVDs are ripped to MP4 and added to the My Movies in Navigator will it play them like the Dune player does?

Yes it has the ability to play MP4.

 

Just keep in mind that like any other container, there are multiple codex for all of them, and occasionally you may find a/some files it can't play.

 

Anything done with a common/mainstream media program without heavy customization should work.