My kids are 3.5 years apart, and I LOVE it. I think it is a perfect gap. My oldest one was old enough to understand what was happening. He was really excited about his sister coming and really invested in her arrival. He is old enough to be really helpful with her. Like I can leave the room for a few minutes and know that he will either stop her or let me know if she is doing something she shouldn't. He takes her down slides at the park. He helps her in the car seat while I am driving.
Yet they are still close enough in age that they really enjoy each other. They dance and chase each other, jump together in the trampoline, swing side by side at the playground. Of course there are things that he likes to do that she is too little to understand, but I expect that would be the case even if they were only two years apart.
Conversely, my sister and I are 21 months apart, and I just felt we were right on top of each other all the time. She was always bugging me, and we fought constantly. I felt like we didn't really have space to be ourselves. It wasn't until we started going to different schools that we started getting along.
What it really comes down to is that you can only engineer so much with your children. They will get along or they won't, and I think ultimately the age gap won't be the determining factor.