I'm hoping that he does, as he is starting to crow and it sounds terribleAt 4 months they want to start breeding. Hopefully he will go after the laying hens. The hens may not be aggressive now, but with new comers there attitude might change. Hens can be more aggressive with new comers than roosters.

And all I can rely on is your experience to gain my own experience (or something along those lines)Given the choice, I believe I'd turn 'em out in the run, assuming it's a large one. As I said, that's what I did, and it worked, but my run was about 12x20, and the smaller ones had room to run away from the older ones if need be. I'd do it first thing of a morning; ideally, they'd get things sorted out before the whole gang was confined in the coop all night with nowhere for the youngsters to run. That could be a blood bath. My youngsters were 16-18 weeks old when I put them in with the hens; a bit older than yours. Also, someone mentioned that having a rooster present was a good idea, too. The first time I brought young birds into an existing flock there was a rooster with them; the second time there wasn't. I had no problems either way. Don't take any of this to the bank; just 'cause it worked for me doesn't mean it'll work in your situation; all I can fall back on is my own experience.
So throw the newbies in the coop after the older girls go to bed?Putting them in at night is the best. Then they wake up together. Confuses them.