I can't imagine that Canon would offer a T4i as their biggest MP camera, ahead of the 1DX, 7D and potential 5DIII, which all seem to be aligning at 18mp.
The difference between 18mp and 21mp is pretty slim. No one will notice any real world difference in their prints. I could quite happily run a 5DIII or 1DX along side a 5DII and no one would notice the MP difference and as I mostly use fast primes for my professional work, I doubt that anyone would notice the lower iso noise ability between the two cameras either....so why upgrade you might ask? It's quite simple really, shutter count on the camera is getting quite high and one of my 5DII's is about to enter it's 4th wedding season next year. I worry about reliablity.
There is a difference on the way that a full frame/x1.3 sensor camera handles ISO noise over a crop sensor camera as well