I have 12 months between my first two girls, and then 4 years between my second and third. Its still early days, with DD3 only just turning 1, but they get on incredibly well. DDs 1 and 2 have an incredibly strong bond at the moment and have done their whole lives, but they adore their baby sister and the three (or any combination of two of them) play together very well.

I too had/have concerns that my baby girl will be a bit left out of her sisters' fun - I have no doubt that in years to come the 4 year age gap will result in a very pesky little sister, but equally I'm sure that as my older girls age there will be issues due to them being so close - mutual friends, boys (heaven forbid!!), space to themselves for one when the other still wants the closeness ITMS.

I also agree with PPs that it will come down far more to the personalities of the kids, rather than their ages. I have one brother, but we have never got on. I guess if we had another sibling one of us would have been left out, but as it was we were basically both on our own anyway.