I am integrating a 4wk old with my 3 6weeks olds tonight, and have been reading alot.
You can put them in the coop but separated somehow for up to a month so they get used to each other. I am not patient enough for that so I'm gonna try to just go for it and if that doesnt work then I'll back up and take baby steps. All mine come from the same place, a small feed store, and the chicks and pullets are always healthy from there so I personally choose to risk not quarantining.
A LOT of advice says you should put the new ones in there at night, and supposedly in the AM they wake up together and don't make too much fuss. It will probably be hard to sleep wondering what is going on out in the coop, but I'll try it.
I'm going to put my new girl in the coop tonight past 10pm (because this is well past sundown, they'll all be zoned out) and try to get up early and make sure they're not making her bleed or being real cranks to her. When I go out there in the AM I'll be bringing grass bits, oatmeal, and dried mealworms to scatter in hopes that they'll get so excited about snacks that they bond together. I will bring a stick with me and sit in the run for a bit and if they try to peck her too much I will "peck" them with my stick, this was another idea I read.
Because your chicks are older, there might be more fuss. But if they're all sort of the same size it should really be okay, I think the "fatal" fights happen with very young chicks going in with older ones. I think its important to stay calm and assume everything will be fine, I don't want this "chicken thing" to start being a pain, I just want some cute backyard pals and EGGS!!
I appreciate post updates myself, so I'll be sure to update on how things go tomorrow!! Wish me luck!