I have 4 who are all 2 years apart in age (except the youngest 2 are twins). My oldest 2 boys shared a room together before the twins arrived and often go to new activities together since they are relatively close in age. However, very often, the youngest three (DS2 and the twins) play together and leaved the oldest out. Most of the time he prefers it that way, but sometimes not so much. But now that they are getting older I'm starting to see similaries in personalities between the youngest twin and my oldest son. They are more introverted. And the girl twin and DS2 are very similar and play together really nicely.
Op, sorry if that was confusing but to answer your question if a 4th will round things out, honestly it depends on age gap between the kids and their personalities. You have a better chance of them all playing together and partnering up if the age gap is smaller, imo. Oh, and yes, it is very, VERY busy around here.
" I object to violence because when it appears to do good, the good is only temporary; the evil it does is permanent." Mahatma Gandhi
"This is the ultimate weakness of violence: It multiplies evil and violence in the universe. It doesn't solve any problems." Martin Luther King, Jr.