I don't care what their rationale is, the version numbering ideology of the mozilla foundation is dumb. There's not a good enough reason to stick to version 3.7 for a "version 4" beta, it's super annoying and makes them look amateurish.
Starting with beta 1, there will be no more references to Firefox 3.7. As we speak, all version strings are being updated to refer to 4.0 instead.