You may have to build kennels if you have not already. It is a very bad idea to leave four Pit Bulls loose in a fenced yard together. (This is assuming that is what your setup is like. I have no idea.) There are lots of problems in addition to the potential fighting. Theft. More pregnancies. Escape. Tampering by nosy/vindictive neighbors. Calls to animal control. Even if your current dogs are dog-friendly, the more you add to the "pack," the more likely they are to "gang up," and that can lead to fence-fighting and redirecting behaviors which are no fun. That is not to say they can't possibly all hang out together while you're around, but you don't want them to all be loose together unsupervised.
There are a lot of elements to dog aggression. One is simply that some dogs like to fight, and there's not much you can do about that. You can try being a strong and dominant pack leader, but you'll have to constantly be on your toes with them. Then there is aggression over food or toys, which would require you to pick all those things up when the dogs are out together. There is also redirecting aggression (mentioned briefly above) where a dog wants something sooooo bad that when he can't get it, he grabs the closest dog to him instead. Unfortunately, if you own Pit Bulls, aggression is part of the big picture.
It does not really matter if you keep males or females. I've seen equal aggression from both sexes. A lot of people say females have an easier time getting along. In my experience, males are more mellow and females are more bitchy. But on the other hand, when males throw down it can be horrific. Among even my own personal dogs, I've seen males and females fight each other, too.
If your dogs are already outside dogs, have you thought about where your female is going to whelp her litter? She will need a safe, secure place away from the male and the general public. Even in setups where all the adult dogs are on chains, most breeders still have pens where the mom and her new litter are separated from the rest of the dogs.