If you have the newest version of Quicktime and you are having trouble seeing mp4 files, use this tutorial. (Install packages usually don't let you downgrade, so you have to bypass this with a special program.)

http://www.macfixit.com/article.php?story=20080129095056351

This tutorial will help you downgrade Quicktime using Pacifist.

• QuickTime 7.3.1 for Leopard [10.5.1]
• QuickTime 7.3.1 for Tiger [10.4.11]
• QuickTime 7.3.1 for Panther [10.3.9]

(Download Quicktime 7.3.1 for your OS, download Pacifist, open Pacifist, drag the .dmg of Quicktime on to Pacifist or press "Open Package," once the package is open.. highlight the name of the package and press "Install" at the top.. follow the on screen instructions.)