I am the oldest out of 3 girls. I had my own room and my younger sisters shared a room but that didn't occur till I was about 10! I would have really been upset if my parents had forced my middle sister and I to split! I am so close to my middle sister because of the years we share that room. By the time I was about 9-10 I was ready to have my own space.
As for kindergarten - maybe you're thinking she could do homework in her room and not be disturbed? She isn't going to be doing homework by herself in her room. She's going to need to be sitting at the kitchen table with you helping her. The tricky part of that is keeping the younger ones busy and quiet while you're helping your oldest.
True, by having her own room maybe she would get a better night's sleep and therefore preform better at school but since they have been sleeping in the same room for a while now that shouldn't be a problem. I would see the problem being with your 3 year old not getting a good night's sleep for a while with a 4 mo old in the same room!