I got interrupted when I was posting, so I left out part of what I meant to say. I agree, Lynda B, that it always depends on the personality of the individual Betta in everything. Every Betta is an individual and has it's own personality.
I had 4 in a sorority once and they did well.
Nevertheless, 5 will usually make it. There is a larger percentage of sororities that will not do well with only 4. Stay around the forum long enough, and you will see this come up repeatedly: members trying and failing with 4 or less girls. So I will usually wait until I have at least five to start a sorority. I had to treat a girl awhile back. I put everyone out of the tank into single housing until I had the five again. I would like to add a few more girls. When I do, I will take everyone out to separate housing and then rearrange the tank before I add the new girls and then only after a quarantine.
Even so when I set this one up, one girl had new nips on her fins daily. The two pk girls were older had been in a sorority before and told that little girl off frequently as she is a stuborn little one.