Same idea as the triangle, but maybe the fourth person is just standing back and unknowingly making things worse. In being supportive of the other two people against the one messing things up, this fourth person keeps bringing up past injustices and other things. If you are the fourth person, it is a good thing to shift your angle, move over and let it go. This is not easy. Start by just not talking bad about the offending person. That can be hard enough, but effective. Even harder is praying for the one messing things up. It may not change them, but it will change you. If you don't pray, at least do not send negative energy their way. It is not easy, but I do believe it works.
Just remember, adjusting your angle forces change. It may not happen quickly, but it is a start. Try hard to let go and go on. Good luck!