I would not lock them in the same coop for a few days until they were being nice to each other in the run.
Being as you have a couple of coops, it will depend on your numbers. We tend to think all of nothing, when little adjustments can work best. Put the younger birds in the new coop/run first. Leave them there a few days. Let them get used to it, and claim the territory a bit. Do put up hide outs, multiple feeders, and roosts in the run. Add a lot of clutter, so birds can get away from each other.
Then add the older birds into the new coop. Chances are, there will be one or two real meanies. Pull them back out. Leave the older birds that tend to be more mellow. Wait a week or so, then add back in the meanies...see how it goes.
The older birds will be dominant, but the 4 month old bird must be close to laying, and once they are, pretty equal to the old birds, therefore the slight advantage of being in the new coop first, should even things out.
Mrs K