I'm building a 4x8 coop too! My run will be about 20x30. I'm getting 8 chicks in May. That gives then 4 square feet a peice in the coop (with room to add a couple I think). I did an external nesting box on the short side w 3 cubbies. They are about 12"x13" and 15" high. You can see my build so far under the thread name '4x8 coop'.
The 4 square feet coop space and 10 square feet of run space per chicken is what I would call minimum space requirements. The more area you give them, the more contented the chickens will be. For some breeds the rule of thumb works great, for others, not so well. Personally, I wouldn't have more than 6 average sized hens in that size coop for long term living quarters.