Did you get the mp4 track from the iTunes Music Store? If so, it's DRM'd and you'll need to search the net for a utility that will strip the DRM for you before you'll be able to convert it. Either that or burn it to a CD and then import it back into iTunes as an MP3. That said, if it's not DRM'd, you can convert it to mp3 from within iTunes. Just check your Preferences under the Advanced > Importing tab and make sure mp3 is the preferred format. Then highlight the track you want to convert and from the main menu click Advanced > Convert Selection To MP3. Should create a duplicate of your song in mp3 format right there in iTunes for you, which you can then locate on your hard drive and use.
jag