Most people will say 1, the reason why is because like males the females are just as aggressive and are territorial, for a successful sorority the reccommended tank size is about 20-24 gallons but that said I have a 12 gallon with 7 females and all is well, at the end of the day it depends purely on the temprement of the fish
Actually a 10 gallon(9 gal is close enough) will work just fine as long as you have plenty of plants and hiding spots for them to have territories. Minimum of 4 is recomended. If you only have 3 then theres a chance of 2 of them teaming up on one. Read through Dramaqueen's sorority post http://www.bettafish.com/showthread.php?t=29402 It should answer most of your questions.
It will all depend on the temperament of your girls. I started my sorority about 10 days ago after my 10gal finished cycling and the next morning had to separate one of them, Princesa the CT, because she was overaggressive on the other 3. Given that I had my 2.5G tank in use then, I put a divider on the 10G and isolated her there where she was able to flare at the rest but nothing more.
Three days ago I woke up and Princesa was nowhere to be found on her side of the tank... She had jumped the divider and joined the sorority with ease. She still chases the others around but no more flaring and biting. So I decided it was time for the final member as my filtration and cleaning schedule would allow up to 5 Female Bettas on the tank so I went and got a nice silver and violet VT, Vio and had to watch over the restatement of leadership on the tank. But everything was over in a couple of hours and now the five girls live together in harmony. There is chasing and the occasional drive-thru flaring but that is the end of the aggresion between them.
So, yes you can have four girls on that tank as long as you take the time to get them to know each other and watch for excessive aggression.