I know other post have said that the mini connect will play MP3 files off a USB, but does anyone know if it will play MP4 format. I thought I would ask before I start converting the files. I am still a week out from having my new Mini and just getting things ready.

2014 Cooper Hardtop "Sheldon", Deep Blue, White top/mirrors/bonnet stripes, cold weather and wired package, fog lights, off-white interior, and storage package.

 

Are you talking real mp4 files? Video ones? Or m4a (Apple Lossless)? If video: when talking mp4, the codec tends to matter a lot for compatibility.
But in general, yes the Mini will play video files so long as the car is in park. It will not when the car is in gear.

Not sure on the m4a files though.