Yep, you need at least 4 females. People say odd numbers are better, though. I myself have 4 females in a 10 gal and I'm planning on buying another one later on.
One thing that I noticed is that I bought 2 at first together, they were sisters I think. I had them for a while before I added 2 more, and they got along great. I don't know if all siblings are like that, but I wouldn't try my luck.