Honestly if they have been slowly accustomed to the dropping temperatures, meaning they have been outside when it was 90 then going down from there (even if it jumped from 80 to 60 in a couple of nights) they would be fine because they are 4 months old. I started bringing my chicks outside in the spring when it was starting to go up to the 70s from the 60s and only brought them in at night, then kept doing that so by the time they were a month old they could handle high 60s pretty well.