You could *technically* try 4 females in there together and hope you get some very passive girls. You'd have to plant it pretty heavily and give them plenty of hidey spots. I started out with 4 girls in my 10G tank and they got along just beautifully. then I upgraded to 5, then to 6. Now I'm maxed out in there at 6. Luckily everyone has come to get along again.
I probably would be leary to give the 4 girls in that tank a try, though, because usually that doesn't give you enough to spread out the aggression very well.