Four is fine. I have had a sorority of seven females, before. I started out with three, when I was given the wrong information. Then I got a fourth one, later two more and then another one was added. That was my first sorority, and my first successful one.
The tank is a 10 gallon, right? That's the minimum tank size for a sorority. You could fit 5 girls comfortably in a tank that size.
Are her fins just torn? Or does she have wounds on her body?
The fins will grow back with clean, warm water. For the body wounds, those should eventually heal aswer. Just keep her seperated, until you get the sorority situated or if you want, until she heals completely.
What you can do, to prevent an infection is to add aquarium salt into the water.
1tsp per gallon, added very gradually. The treatment can only last 10 days, any longer and it can harm her organs. Then you can start gradually taking the salt out. Make sure to have the salt dissolved in the water before Cleo gets added in.