What does your run look like? And how much space do you have? Measure it. Often times on here, I will see a run that is just an open rectangle. This makes the run nearly 2 dimensional, and really wastes the third dimension of height.
In a run, where the chickens should be spending most of their day, there should be places where a bird can get out of the sight of another bird. When two birds meet, they often determine who has dominance, with the dominant bird standing her ground, and the lower bird moving away from her. In a smaller run, there often is not enough space for this bird to move far enough away to show submission, which can upset the dominant bird into a harder stronger attack to prove her point. If the submissive bird disappears into a hide out, the dominant bird is satisfied and peace is in the flock. She does not have to stay disappeared, generally I have seen my lower birds, run away, out of sight, the dominant bird goes back to eating and a few seconds later the submissive bird is back beside her, no problem.
Your younger birds are the submissive birds until they begin to lay. So in your run you need some hide outs. Pallets are cheap and excellent, one can wire them up right in a corner, just make sure there are two exits, so as not to create a trap. Place a pallet up on cement blocks, so the smaller birds can get under it. The bigger birds will fit, but it will slow them down. Or put a pallet up of saw horses, so that birds can get under it in some shade or on top of it to roost. Set up some roosts in the corner, my birds love to roost in the late evening sun or early morning sun. And put a pallet up vertically in the middle of the run, Use some fence posts to hold it up. And put a feed and waterer behind it, so that a bird can eat there out of the sight of the main feeder.
As for space, it might be that your run/coop is not big enough for 4 birds, but I think it is because the little ones just have no way to show submission. Now after you make these changes, let your older birds out in the yard for just a bit, put your younger birds in the set up, and then move around in there, toward them so that they find the hideouts, then let the older birds in, they should come right back in with some treats. If there is still problem, put those older birds in the chick pen, and let the chicks in the bigger space for a couple of days.
Good luck,
Mrs K
