My kiddos are 4 years and 3 months apart. My daughter is now 8 and my son is now 4. I really thought it was a good thing. Like you, if I had gotten pregnant when I wanted to, they probably would have only been 3 years apart, but life had to wait. The pros were: Our daughter was reasonably self-sufficient at that point, so she could entertain herself when I was nursing or putting her brother down for a nap. It was good for me to have a fun four-year-old to hang out with when her brother was sleeping. And, though he wasn't a good napper as an infant, it didn't matter as much as it did when I only had one because I wasn't having "me time" anyway. Our daughter really didn't experience jealousy when he was a baby, though now she frequently thinks he gets treated more leniently. Despite their age difference, they play surprisingly well together (that said, they also fight). Another pro is that our daughter went into school at 5 and as a stay at home mom, I was able to spend some one-on-one time with my little one.
Cons are: There is no simultaneous napping, just when your four-year-old is becoming able to do more fun, crafty-type projects, your crawler/walker is going to want to make a mess out of it. I don't think we painted at all during this time! When he was a baby and she was four or five, they didn't have much to fight about because they weren't playing with the same toys at all. Now that they're older, the fighting can be difficult. When our daughter hits back (hard) she thinks their punishment should be the same even though, at twice his age, she should know better.
Over all it was a great age split. Everything I've read says that whether siblings get along or not has nothing to do with age and everything to do with personality, so cross your fingers... and enjoy your pregnancy!