Is the coop inside or outside of the run space? For 4 birds, 8x8 would certainly be respectable. The coop inside the run would give them some shade if you did it elevated like you are showing here, but come at a cost of that 8x8 space. You're definitely on the right track in thinking this through ahead of time. Certainly agree with others that you really would only need 2 nests. I have 8, and they share a medium sized roll-out nest box. I've had 2 in mine at one time, and they manage. Also, the size of the coop is a factor, see about bumping out on one end or side for your next boxes. Then you maximize the space on the inside for the birds. Finally, what are you thinking you'd use the storage for? I'm thinking unless you have specific things you want out there easily accessible, it might be better to go without. I certainly don't know your goals in that area, so storage may make a lot of sense, but if it is easily carried stuff, again more room for the birds would be my first choice.