Posted 15 May 2008 - 04:39 AM
The best software for doing it is mounted between your ears i.e if you really want the best results you need to understand a little about the native mp4 codec you are using and how it differs to the one you want to use. The primary difference is compression level, obviously, but colour management and other factors come into play if you want it to look exactly the same. An mp4 file squeezes the file harder by losing any information that looks the same from one frame to the next very effectively, but it can also lose some of the detail within that by considering small areas of difference as broadly similar. Bottom line is the end result will only be as good as the original allows.
mp4 can also adjust audio compression along similar lines, although this is not as noticeable to the ear depending on the type of sound it is converting e.g speech will sound exactly the same but a full orchestral performance could sound noticeably different. This is often used to save a bit of space in large files where sound is not an issue.
Anyway, there are loads of free converters available on the interweb that will do the job. Which one works best is a case of trial and error until you get one you are most happy with. If you want complete control of the process shell out a few quid and buy a decent basic editing package, that way you can tinker with it and may be able to improve it by using noise reduction etc etc.