Depends what streams are in the mp4. Could be it contains MPEG-4 Part 2 video and mp3 audio, in which case you can just remux. Could be it contains AVC video and aac audio in which case you need to re-encode. mp4 is just a container.
Thanks for your reply: G Spot says the following: Codec avc1 Name H.264/MPEG-4 AVC Container: File Type: MP4 (.MP4) [ISO 14496-14:2003] Mime Type: video/mp4 Recommended Display Size: 320 x 240 Created: 2005 Nov 13 10:08:15 Modified: 2005 Dec 10 14:08:26 Audio: MPEG-4 AAC LC Thanks Again