I usually wait until 10 weeks to integrate, but I know some people do it earlier. I think the 4 week olds would definitely be a little too young. Here's how I do it... I set up a separate fenced area in the run, so that the hens can see the chicks and vice versa. I put them in there during the day and put them back in their separate cage at night for about 3 days. At the same time I do some supervised free ranging (the hens seem to be less bothered by the new girls when they are free ranging as opposed to in the run/coop). If all seems to go fine, I do a supervised time together in the run, making sure that the chicks still have a place to run for safety that the hens cannot access. Keep a close eye on them. If that goes well, I put the chicks on a low roost in the dark one night. Get out to the coop early that first morning, though, to make sure it's not a blood bath!
I may make this a little more involved than others, but I would just hate to go out and find a dead chick! If you ever get a broody hen, let her raise chicks. It is SO easy...she integrates them into the flock with no fuss!
Good luck!
