Once they are close to fully feathered, they can go outdoors, but if they were my chicks I would wait until closer to 8+ weeks to put them where the older chickens have access to them. Make sure they have a good temporary coop, especially if you are somewhere where it is cold right now. You can set them up in their own area where the other chickens can see them but can't get to them. After a few weeks of looking at each other and interacting through the fence, they should be ready to live in the same coop and run. The older chickens will still pick on the young ones, but they should be big enough to defend themselves as long as they have enough space to run and hide. Multiple feeders/waterers also helps minimize conflict.