First, I suggest you follow the link in my signature below to get some of my thoughts on why chickens might or might not need more room. We are all so different we have different requirements. There are different factors in that. Some of it is to have enough room that they don't bully each other or even turn into cannibals because of crowding. Part of that is that the less room they have the harder you have to work or the less flexibility you have when things happen. My stress level really drops if I have flexibility when I need it.
Chickens don't see space as "coop space" or "run space" or "free range outside space". They just see it as space if it is available when they need it. If they are locked in the coop only for a time when they are awake coop space is much more important than if they can get outside as soon as they are awake. It may be hard to get them all to go into the coop at night if it is too crowded. Where are you located? What will your winters be like? Will your chickens be locked in the coop for days on end because of snow and the weather? That could cause problems.
One way chickens have learned to live in a flock is that if there is conflict the weaker runs away from the stronger and then avoids them. That's not a "square feet per chicken" thing, that's a "can it get away and avoid" thing. In a 4x8 coop there isn't much room to do that even with just two chickens. The reason a coop like that usually works is that if you have a flock that are fully integrated and are at the same level of maturity, there generally isn't conflict. When you integrate new chickens, especially with a difference in maturity, you need a lot more room because there will almost certainly be conflict. If you let a broody hen raise chicks with the flock Mama needs enough room to work and if they are crowded there probably will be conflict. After Mama weans the chicks and leaves them on their own to make their way with the flock the immature chicks need to avoid the adults.
With your 9 all being the same age and being raised together I think there is an excellent chance they will be Ok in your coop together though a rough winter could lessen those chances. If you had more than one boy you could have issues. If you hatch chicks you will probably have boys. Occasionally you get a girl or a boy that is a brute, I don't keep those. I can't give you any guarantees about any living animal and behaviors but I would not be that concerned with your current set-up and all girls unless your winters are harsh.
I would not try to add any more chickens to your flock and expect them to also use that coop. That doesn't matter whether you get chicks or let a broody hen raise them and try to integrate them for you. You might be OK with a broody hen until she weans them or tries to take them to the roost but the chicks would not have any room to avoid the others then. That's not because of any mythical, mystical, magical square feet per chicken number but simply because you don't have the room for them to avoid the others.
