I have an MP4 in an email that I want to put into my iPad's music library, how do I do that?

Directly on the iPad, you can't. you will need to get it on a computer than sync it via itunes from that computer.

alternately, you can download an app such as VLC from the app store and transfer the file into it.

You can't copy it into the Music app directly on your iPad - you will need to open the email on your computer, copy the track into your computer's iTunes library, and then sync it from there to your iPad

there is an app called video download that will let you watch mp4. If you install it you may be able to tell it to open that file with it and view it