4.0.3a, the Shattering, is not an in game event. The Cataclysm world changes happen overnight. This IS Cataclysm.
4.1 is confirmed as the Firelands/Abyssal Maw content patch that will come many months later. Wrath was 3.0, BC was 2.0, etc, no other expansion started at X.1 that I know of, I don't know why you would think this one will. 4.0.3a will BE Cataclysm, we just can't access the new content until Dec 7th but it will be there more or less.