Whose MP4 files are you attempting to play?
I'm trying to play Kaizoku-fansubs' version of One Piece. They've recently moved to MP4. The audio codec they're using (AAC audio codec) is mentioned on their website; I've downloaded, extracted and registered it.
Here's the Test Render log of the MP4 movie file:
Test Render of: C:\Documents and Settings\---\My Documents\Anime\Series\One Piece\[K-F]_One_Piece_167_[13B38BA3].mp4
Filter 0: Video Renderer
Filter CRC: EF5A266F
Date: 2005-08-29 | 23:54:26
Filter 1: ffdshow MPEG-4 Video Decoder
Filename: C:\Program Files\Combined Community Codec Pack\Filters\ffdshow.ax
Filter CRC: 7A26452F
Date: 2005-12-19 | 21:38:20
Filter 2: C:\Documents and Settings\---\My Documents\Anime\Series\One Piece\[K-F]_One_Piece_167_[13B38BA3].mp4
Filename: C:\Program Files\Common Files\Ahead\DSFilter\NDParser.ax
Filter CRC: 1528FBF8
Date: 2004-07-07 | 14:34:40
Hopefully I did that right.... I'm a little confused on what you wanted me to "Test Render." I assumed you were referring to the MP4 movie file.