I have a 4 year gap between my boys, and it's been great. My older son is able to entertain himself while I feed the baby, and is more independent. The small gap would be too much for me, particularly as I had morning sickness all through my second pregnancy.
The downside is that as they get older, the gap means they probably won't have as much in common to play.
I know a couple of people with 4yr gaps, and they seem to enjoy the benefits as well.