The female of the species is not unlike the junior high version of humans! You should see what a tankful of them will do to each other! Anyways, if you introduce hiding places, like plants, you can minimize the damage.

Another consideration is that not every short-finned betta is a female!The surest way to pick a female is to look for the stripes along the side of the body.If the fish is a solid,bright color the chances are fair to good that you have a male.If you want to minimize agression putting two together is not the way.Better to put all four together from the get go,assuming that they are all females.

I have bred bettas for almost 9 years and I've found that once jarred, a female usually can NOT go back into a community setting. I've raised a tank full of females (from egg) with LOTS of plants/hiding places and they still fight, though not as bad as males. They are still siamese fighting fish. If you do not buy the females together, please do not put them together because I've had females outright kill each other when they've jumped out of their tank, and I found out the hard way (in the beginning) not to put a jarred female back into the community tank, lest she kill or be killed. It's not worth her life. If you don't want the females, take them back to the store or find someone who will take them and care for them correctly.

Here is a link that might be useful: Bettagurl's Bubblenest