MP4 is a bit different in that it's a file container capable of having video and/or audio embedded in it. The audio track to MP4 files is often a .aac file (Advanced Audio Compression). With the proper software, you can convert the audio track of a MP4 file to an MP3, but then you're taking a lossy compressed format and converting it to another lossy compressed format, which will further degrade the sound quality.
MP3 files are the audio tracks to MPEG movie files. MP3 stands for MPEG Audio, level 3, I believe.