If you can get electricity to the coop, put in a heat source in one corner without burning the coop down and move them out now. My brooder is in the coop to start with. The chicks go in there straight out of the incubator. I heat one end of my 3’ x 6’ brooder and let the rest cool off as it will. Even newly hatched chicks know to stay where the heat is, and in a few days they roam a lot more and return to the heat when they warm up. At four weeks yours should be really good at self-regulating. You might be shocked at how comfortable they are in cooler areas, just going to the heat if they need to warm up.
If you don’t have electricity you can try building a hover. Think of an inverted flat box raised just enough for the chicks to get under it. Their body heat warms the inside and with the roof on it, they stay really warm. Don’t be shocked if they sleep on top of it instead of under it though.
George is right that if they are feathered out they should be OK. I’d be a bit concerned at that age of taking them straight from a 75 degree environment to a freezing environment. You might try exposing them to cooler temperatures to help get them acclimated. Make sure the coop has good draft protection where they are. Ventilation up high is really good but they don’t need a breeze hitting them directly.
In the heat of summer I’ve been known to stop the daytime heat at 2 days and nighttime heat at 5 days in my brooder in the coop. In winter I leave the heat on much longer. As a general guideline if the lows are somewhere in the 40’s and they are five weeks old, they don’t get supplemental heat. I’ve had them go through lows in the mid 20’s when they were 5-1/2 weeks old and do fine.
If you have been feeding them a fairly high protein chick starter they should be pretty well feathered out by now. With just a little acclimation they should be real close to be ready to go. It’s hard to give real solid advice over the internet because no matter what you tell me I’m not looking at them, but they are normally a lot tougher than many people realize. I let the chicks tell me what they need more than go by any rigid schedule.
Good luck!