You don't necessarily have to keep them in the brooder that long. I usually judge by outside nighttime temperature and feathering to determine if they are ready to be outside. Your 4 week olds probably went in just fine since they had no competition but introducing birds to one another is a...
Here's what has worked for me;
I leave them in the brooder until about 4-6 weeks. Then i take them outside and i do 1 week of see but don't touch. On the 2nd week i let them free range together (supervised) for short amounts of time each day, getting longer as the week goes on. Then i judge how...