I have a small aquarium with three female bettas. They all get along together. I added a fourth female betta and it has become aggressive to the other three? Should I remove it? What could happen if I leave it in the tank with the other three? Could it kill one of them?

Yes, it could kill all of them.
Some female betas are more aggressive than others.
If this female beta is aggressive and picking on the others,
She could very likely kill them all.
It would be wise to remove her from the tank.