I found these plans free online. I'm getting around 4 chickens and have decided to make my own coop, the kits you buy are garbage. This looks sturdy and easy enough to build.
Dunno why they have 3 nest boxes for a 4' x 4' coop when only one is needed for 4 chickens.
The floor of the coop I would use a 4'x4' piece of plywood and pressure treated 4x4's for the legs. For a 4' x 4' floor 2x4's would be fine for the floor frame, I'd add one across the middle.
The plans don't show any vents.