Since you are out of town on long weekends or something that huge run doesn’t count since they don’t have access. You are limited to the 4x10 area. That 15’ x 50’ area is great when they can use it but it does not count when they can’t. It is not just about space, it’s how you manage that space.
Modifying your profile to show your general location can really help with questions like this. If you live where you never see snow then the comments about snow don’t mean much to you. But something else might. Your general location is often very helpful information to have.
I don’t believe in magic numbers for space or much else to do with chickens. If you follow the link in my signature you may get an understanding of why. With my understanding of how you manage your current set-up I would not recommend adding any more chickens. If they had been raised together you could probably have another chicken or maybe even two more in that space, depending some on your climate and some other factors. But they were not raised together, that 4x10 is really small to go through an integration. If they are getting along in that now I would not mess with it.