[metacorrector]

mp4 actually tells you more about a file than mpeg4 does, because mp4 is only one thing, the file. mpeg4 is another thing, but that thing is not a file. So, calling files mpeg4 actually confuses things rather than clarifies them. If you work with these concepts, it's nice to be able to say to your coworker, "hey, send me the mp4". if you say "send me the mpeg4", you'll get back "how do you want me to send it?"

This works for many similar concepts. The idea of not overloading words that already have meanings with other meanings was one of the sensible underpinnings of the Hungarian Naming Convention. Let's not rehash the Hungarian debate, you don't need to like Hungarian to realize the sensibleness of this particular concept.