I really think it's individual.
My sister and I are 18months apart and while we get along OK, we aren't close. My brother and I have a 5 year gap and we're much greater mates, but we're much more similar personality wise than my sister and I.

Mine are all 3 years apart (well DD-DS is 2y10months, DS to belly bub will be 2y11.5months).
Not at all planned. We wanted them all close together and started trying at 6months and 12months post bub. Just not to be. In hindsight I'm grateful. I couldn't have managed with 2 littlies at once.
I got time with them alone that I wouldn't have had if they were closer together. They were TTed before their sibling arrived and are great little communicators. They're reasonably independent. That works for me.
They're still great mates. DS learns so much from DD, and she learns a lot from him.
TBH if I'm lucky enough to get a fourth, I'd want the same.