My gaps are 11 months, 3 years, 13 months. Currently have DD1 - 5, DS1 - 4, DS2 - 15 months, DD2 - 2 months.
Agree with lollystix - it is hard fitting in kids with small age gaps and work. I have gone back between mine, but with the 11 and 13 month gaps, went back pregnant, and only stayed about 4 months inbetween! I have just had to get used to the constant jokes and office banter....
On the plus point, with the smaller gaps, they are so close, and play brilliantly, so although the baby stage is hard work, once they get a little bit older they more or less entertain themselves.
The hardest thing I have found with our 4th baby is trying to get 2 very little ones out the door in the morning for the school run, which I obviously never had when my first 2 were small.
Good luck with whatever you decide works best for you!