They don't need it and they won't use it anyway if they are roosting. If they are 4 mo. old now, they've developed the feathering to withstand cold temps and will do just fine. Resist the temptation to close up the ventilation on their coop, as that will just make them colder as the humidity from their bodies and breath collect on the exposed areas of their body and on their feathers, creating a chill.
Just give them nice, dry bedding in the coop, plenty of water, and good food. That's all they need to get through this cold. People who lose chickens during the cold of winter had unhealthy chickens to begin with, as cold does not kill chickens, the stress of cold on an already weakened constitution will kill chickens.
Chickens are imminently suited for cold weather outdoors because God provided them with downy feathering underneath their guard feathers, both of which trap heat to their bodies. It's 11*~with a windchill of 2*~ here right now and my chickens are out wandering around in the snow after spending a night of negative temps in an open air style hoop coop. They are very content and comfortable, not huddled and shivering and trying to stay warm.