You didn't mention the amount of ventilation you are or have already installed.  What are your plans for your girls during prolonged winter storms?
Also, where will feed/water be located? That will be very difficult w/ such a small amount of space for four large, fluffy girls.  Even Orpingtons turn on each other in such a small space, if have to be cooped while they are awake during storms.  
I had 2 Orp hens & an Orp roo in a 4x8 coop w/ a combo of movable fencing & free ranging.  They were NOT my happiest breed of birds in such a small coop area.  I did have feed & water in the corner next to door & nestbox over the feeder.  In good weather, the feed & water moved out, but in bad (hurricane, ice/snow storm), when birds were penned (open air CP hoop coop), the feed & water took floor space.  When i changed types of feeders/waterers, taking less space as hung from wall, but it didnt really matter.  They just didn't do as well in that size of coop & it's 2x the size you're working with.
That same space is in the works to be enlarged from 4x8 to 8x10 w/ a permanent run that will probably start @ 10x 16 & be able to be permanently enlarged in a couple of different configurations.  I'm not planning on more Orps at this time, but will have larger, meat line of Delaware in this coop to start.  If not large enough, they will move to a 10x16 coop in our pasture (also being rebuilt from an 8x8 CP hoop coop) w/ run that will be 10x16 & attached to larger garden area (seasonal & rotated w/ another coop).