I just noticed your screen name. Are these silkies?
Keep in mind, a mother hen doesn't heat all ambient air, just the spot under her and they do fine in cold. Your birds would no longer be under a setting hen anyway. They aren't that fragile, but I've never raised silkies.
Is a sweeter heater the overhead infrared brooder?
If so, I think they'll be fine. 4 of them under a heat source and at least 2 months old.
There is no wind chill effect inside the building. Wind chill is the effect of temperature and wind velocity on skin. If there are no drafts in the...