They don't need a heat lamp at all at this point - even if it does get "cold" (and that term is relative - I'm in MN, and "cold' to me is entirely different than it is to someone in, say, Florida or California) Your chickens are 4 months old. They are fully feathered, and basically full-grown. If you're just leaving a light on for them for some other reason, don't. It won't kill them - before I got heated dog dishes to use for water in my coops in the winter, I'd have a light over the water dish for them just to keep it from freezing. It was on 24/7. The chickens were able to sleep just fine, but how soundly could you sleep with a light on all night long?