I know this might be an very ignorant question, but do mp4s exist? Because I've heard this name mentioned before. I know that there are MP2 and MP3s (obviously), but I had never heard of MP4s. If so, then how's their sound quality? How do they differe from MP3s? Also, are there any MP4 players in existence (software or hardware). I'd appreciate some information regarding MP4 files (assuming they're not just some sort of hoax) Thanks.