If the key word was "comfortably", then my response would be one or none. It is simply too small for adult birds. An often quoted "standard" is a minimum of 4 SF per bird........and that standard goes back at least 100 years....and was for the house only. Assumption......even then, was birds would be let out into a pen or pasture during the day. And that is interior space.....so for a 4' x 4' coop, use no larger than 1.5" x 1.5" lumber for framing to minimize the area lost to framing. Better still, bump it up to 4' x 6', 4' x 8' or 6' x 6'. Would you believe a 6' x 6' house is over 2x larger than a 4' x 4'? Do the math. It is.
The good news is that by the looks of it, you have the skills needed to build a bigger one......you just need some decent plans and ideas on what to build. If you have 4 birds, you have some time........but not much.