You are taking a prudent path in keeping the 4-week olds separate but in view of the older chickens for a bit until the older hens become accustomed to them. But chickens do not "smell" each other. They instead learn the details of each chicken, especially the faces, and will recognize their flock members. They recognize human family and friends, as well.
It will be very simple integrating them if you keep a safe pen or cage for them to go into that the larger hens can't follow. I call it a panic room. Food and water are inside so the chicks don't need to compete for essentials, and they can escape the pecking order when it gets to be too much stress.
I have eight-week olds who enjoy lounging in their safe pen away from the hectic pace of the rest of the run, even though they're entirely capable of dealing with it at this point. In another couple weeks, they will no longer fit through the 5 x 7 inch openings and will have to give up their safe haven then.
As far as your heat issues, you really do not have any. I doubt you will even need to worry about heat of any sort in your coop in winter, either. Chickens are quite comfortable sleeping in temperatures down to freezing at night, and will adapt to even lower than that.