What media player does one use to play this format?

 

Quicktime or any divx player.

 

Itunes as well. When I import a cd on to Itunes, I know it turns the files into a MP4.

 

That is weird, my iTunes converts music CDs into mp3s, not mp4s.

You can also drag & drop the mp4s into Windows Media Player 10 and they will play.
This is what I do to "test" out my movies that I converted over to the iPod (mp4) format for video.

Right now the mp4 extension is not assocociated with any app on my PC, so I have to drag it to an open player or "right click" the file and choose open with to play it.