I love 4 years apart!
In my family (growing up) we were four years apart (me, 4 years later my sister, 4 years later my brother) and it worked out so well. We played, were very close, not much sibling rivalry, etc. And you don't have to pay for more than one college education at a time!
I liked the fact that my siblings and I didn't have to compete for the same things - spots on sports teams, chairs in band, homecoming court, friends, boyfriends, whatever. I know winning and losing is something everyone must learn but it was nice not to have to compete with them.
I think the spacing of your own family (growing up) and your perception of it as good or bad makes the majority of many people's decisions on how to space their own kids.
Dd1 and dd2 are four years apart - just like my sister and me.
But then we screwed up our four year plan (boy did we surprise everyone) by expecting #3 when dd2 is only 2 and 3/4 years old. We'll see how that goes.... But during childhood I was much closer to my sister (not sure if it was that we were both girls or that 8 years apart was a lot - between my brother and me) but now I am very close to both of them. I just feel that my sister and I had more of a shared childhood. By the time my brother was running around and talking, I was really into my friends and going out of the house to ride bikes or go swimming or stuff that he was too little to do.
I also think that spacing kids four years apart is easier on the parents - emotionally, physically, financially. I understand that parents of closely spaced kids have it harder early on but that they say it is easier after that. Maybe so. I myself liked having the time in between kids to sleep, not change diapers, have the freedom of going out while dd was with a sitter. I adore the baby stage but it is challenging and I don't know that I personally could do it year after year after year if I spaced my kids closely. I also think that for me I would have a hard time perceiving it as fair to the child - I like having the individual time with the first one - then when the first one is in school, you have individual time with the second one.
There are lots of variables to consider....