I put a lot of unnecessary thought into age gaps while we struggled to have our 3rd LO. I don't think that is matters as much as we worry it does. My mom is one of 6 kids and the ones who are closest to each other are not the ones closest in age. But that's not what you asked!
My girls are 6 weeks shy of being 4 years apart. They are really close and have played together all the time since DD2 was old enough to play. They are 8 and nearly 12 now. Things have a been a little different this summer as far as the two of them wanting to do stuff together but I think it has more to do with DD1 approaching adolescence and being moody as hell. If we are camping or at the beach they are great together, but here at home they've been more prickly with each other than usual. However, right now they are making cookies, singing at the top of their lungs and having a great time.
The hard part is that DD2 rarely gets to be the best at something. She gets that her sister has been doing everything longer than she has and has therefore had more practice, but it still bothers her sometimes. Also, DD1 is starting to have more freedom and she and her friends can go off on their own a bit which makes DD2 jealous. As a parent it can be a challenge to keep things fair even though it can't be straight up the same.