Four ft. of roosting space would best accomodate 4-5 chickens. It's generally recommended to give 12 inches of roost space per bird. At times, they'll squeeze together and it'll seem like there's plenty of space. But 12 inches is suggested so that they can flap their way up there, so they can turn around, and so that birds can scooch over a bit if there are personality conflicts.
Personally, I'd advise four birds in that size coop, because of where you're located. I'd guess in your area, that unless you have a roofed run that you tarp up a bit seasonally, your birds will spend (by choice) a decent amount of time indoors over the winter months. The more space the better, especially in areas that can get severe weather.