I was in a quad, but only one married couple was involved, the others were my boyfriend and I. We did not even live together, we started dating just a few weeks prior to beginning to also date this couple. It was really nice for about a year and then came a time where I just felt stretched too thin, and was getting concerned about some communication problems and other issues. Decided to pull back, try to just be good friends (with or without benefits) rather than "girlfriend" to so many.
Meanwhile, the other lady in the quad had been dating other men but the guys did not have other partners.
After I pulled out of it, my boyfriend sought another "primary" sort of person and ended up dating a woman who was really, really not into poly. Like even more than my boyfriend now, Zen, is not into it. Zen is not a very poly dude but he was willing to accept me as I was...this woman was actually averse to the idea touching her life. Had had a bad experience in the past. So Analyst (the former bf I'm talking about) wound up going monogamous with her.
It has now been another...year plus...since then. He's still with her. But now they are playing around with poly again with some other people, I have heard vaguely through the rumor mill. Or at least I think someone said something to that effect...I didn't really pay a ton of attention. Didn't wanna pry into their business.
The married couple, Fire and Hefe, are still solid. I believe they recently celebrated their 15th anniversary. They are still open, she still dates others, and I think he hooks up with other ladies when he can, but I haven't heard that he's formed any solid relationships.
I guess that how this could go down, would have to do with those involved and the feelings with the breakup. If there is tension, or hostility, I cannot see it going very well. And the people would have to have good boundaries. If the breakup were more chill and amicable, just "can't do this for reasons, sorry" then maybe the other unmarried dyad could continue ok.