From what I could tell with DD, there didn't seem to be much difference. A friend of mine said 4T tends to be cut a little bigger in the bottom to allow for diapers. 4 is just the smallest size in the "big kid" clothes, so it's like an overlap from toddler to big kid clothes.
Re: 4T and 4?
DD, 1/7/05 * DS #1, 1/25/07 * DS #2, 11/11/09
Baby #4, EDD 11/11/12
m/c 7/30/08 at 12 weeks (blighted ovum, emergency D&C)