The 4 sf is just a sort of arbitrary number, but when I have more than that, the coop is much dirtier, smellier and just overall harder to keep dry. Too many breathing and pooping in a too small space makes for wet litter. With my BBS Orps, I would never try to have only 4 sf per bird since my rooster alone is almost turkey sized, so in their coop, which is 4x8, I put only 4-5 birds in there; seems almost crowded when they have to stay inside during bad weather.