Not been in one, but it's no different than singles coming together to form something. Once I was the hinge in 3 singles coming to form a "V." There are other singles who come together to form other poly configurations. Whatever works for them is good.
My only advice is to talk about how you want to break up with this configuration ends. (4 people config)
I mean, all relationship come with a clock attached. Your previous 3 people thing ended. It is now a 4 people thing!
Should this break up, will it go back to original (3) + (1) friend? How will the people deal if it break down to all people single again? Or some other combo in between those two places?
I find that not only does talking about it keep it real, talking about it creates emotional intimacy and security which helps strengthen. So the likely of it ending gross
diminishes fears and people can relax and enjoy being in it.
And if they can relax and enjoying being in it... they can handle whatever comes up in Life as a group better.
Which increases their willingness to stay in it, and that increases learning the skills required to keep it going in a good way.
Like a domino effect of sorts?
Not sure if I am expressing that idea well...but that's my 2 cents. Be honest, keep it real.
Maybe something there resonates?