Should I wait till the pullets are closer to laying? Or is 4 months old enough?
People get so hung up on magic numbers, as if they don't live in a reality world. The 4 x 10 space numbers are guidelines. They are way more than enough room for a lot of people but occasionally you can run into problems with them even if the chickens are the same age and already integrated. You are dealing with living animals, not programmable robots. Each chicken has its own personality. People act as if their coops and runs are in separate entities, not combined in their chicken keeping system. When I cook I follow some recipes carefully. You need to get the proportions and timing right when making jelly or baking a cake. But when I'm making stew or chicken broth I don't have those limitations. I follow a basic framework but have a lot of freedom within those guidelines. The guidelines are there to point you in the right direction, they are not laws of nature. Rant over, sort of.
Chickens do not understand the concept of coop space versus run space. The concept they get is "if they need room is it available?" That can be either in the coop or in the run. If you keep them locked in the coop and out of the run then the run is not available. It might as well not be there. But it is there so you need to plan to use it.
Is your run predator safe? Would you be comfortable leaving your pop door open all night, knowing that if a predator got into your run your chickens are definitely not safe? Some people have this which can make this process a lot easier. It affects what you can do.
Your 4-month-olds are not laying yet. My flock and set-up are totally different from yours. I have an 8x12 feet coop and over 2,000 square feet of run space. Until my pullets start laying they form a sub-flock, totally avoiding the adults during the day and not roosting together at night. There are exceptions, there always are, but this makes my process so easy. You don't have this luxury.
It is possible yours will quickly become one happy flock, staying together during the day and sleeping on the same roosts at night. This does not happen often enough to count on it but it can possibly happen. What I'd expect to happen with yours is that the young ones would stay in the coop when the adults are outside or vice versa. They are avoiding them. Mine usually form both of these groups outside but when I have chicks still hanging out in the coop, my hens tend to ignore them when the hen goes in to lay an egg. Not sure what will happen with your facilities and just two hens.
I read a lot of stories on here where a hen will go out of her way to attack any young one. It doesn't matter if the young ones are trying to avoid her, she still attacks at every opportunity. I've never seen that with my flock, maybe because I have so much space, but enough people I trust on here have seen it that I believe it is really possible. You can read a lot of theories why she does that but the theories don't really matter if it is happening.
Those chicks are old enough so cold weather is not a factor, especially with our winters down here.
I just saw your last post about construction. Good! It is possible you could just throw them together and they will sort it out but I would not count on that. I think it is a great idea to have a place where you can separate them immediately if you have a problem. Aart's guidelines for integration work the majority of the time. They are a good outline to follow.
On quarantine, where are you getting them? If they are coming from a flock that has been isolated from any new birds for a couple of months they have been in quarantine. Would you trust the person you are getting them from to tell you if anything is wrong? If they are coming from a place where they have been exposed to outside chickens in the past month or two that is a different situation.
Then you have flock immunities. Some flocks develop an immunity to certain diseases or parasites so they are never affected by them but they can pass that disease or parasite onto other chickens. This could be your new birds, it could be your existing birds. Quarantine is not set up to handle those. One easy example of this is Coccidiosis. Chickens quickly develop an immunity to the parasite that causes this disease so they won't be affected no matter how long the quarantine. When you get new birds it is a good idea to treat them for mites, lice, and worms as these are easy to transmit.
If you wait and get two pullets that are laying they should act like adults. They will still have to sort out the pecking order. Sometimes this is pretty quick and really peaceful. Sometimes it involves fighting. Usually it is over with pretty quickly. If they are not laying yet you may have to deal with keeping them separate during the day and at night for a while.
You are dealing with living animals so you don't get guarantees. Sometimes this is easy, sometimes not. People do this all of the time, usually successfully. Good luck!
