How can I view an mp4 on a Mac?

Use vlc player is probably the easiest way.

Maybe if you install Perian (which I recommend you install anyway) you could play them in quicktime but I never tried since I think vlc is a superior player to quicktime player.

MPEG-4 video files should be natively supported by Quicktime Player. If Quicktime gives you a codec error, you can try installing Perian (which adds more codecs to Quicktime) or VLC (a standalone video player)

Matt