Does that 5x6 count the nestboxes? If the actual floor space is only 4x6 then only 24 square feet counts and the maximum number of chickens should be 6 -- making it generously-sized for the current 4.
You won't need more than 2 nestboxes even for 6 hens. A friend of mine with one of those coops with too many nestboxes blocked the extras off and uses the space to store her feed.
You will need the upper ventilation 24/7/365. Unless you're in the mountains like up around Boone, it never gets cold enough to even inconvenience a chicken in North Carolina because, given their built-in down jackets, chickens tolerate dry cold down to 0F without issues. Heat, however, can be deadly.
Our NC chickens will acclimate to be OK in weather that will endanger chickens in places that are normally cooler, but it's still important to maximize airflow for them -- even allowing gentle breeze across the roost in the summer. That's why my
new coop that we're building is the Open Air style -- essentially a roofed run with a 3-sided shelter toward the winter winds.
Here's a very useful article about extreme weather. I personally haven't had to do anything other than provide shade, copious amounts of water, access to dirt to dig into, and electrolytes once a week, but I made a point of, for the most part, choosing breeds I could expect to cope well with heat.
https://www.backyardchickens.com/articles/aarts-extreme-weather-spiel.75893/
I love being able to keep my
feed and water out in the run. Since the current setup is temporary (a year's worth of temporary as of the end of the month

), the roof is a now-hail-battered picnic pavilion but shade and shelter is valuable. I think that part open (or roofed with wire), and part roofed is ideal so that the chickens can choose where they want to be when they want to be there.
Pictures?
I love blue chickens. I have Blue Australorps among my flock and they're fabulous chickens.