I happen to think that a quad living situation would be ideal for us.
However, I don't think that it's necessary for our lovers to be "shared". While I am bi-Maca is certainly not. Furthermore, we often have different tastes in women!
Ironically, at the moment, he is pretty serious with a FWB. They've been seeing one another when possible (some significant restrictions on time due to opposite work schedules and family responsibilities) for nearly a year now.
I find her AMAZINGLY beautiful, intelligent and adorable. I like her A LOT. I could totally see myself falling in love with her also if we were to live together.
But, I'm not willing to allow my emotions to run wild with someone I know I can't be sure I'll see once a month! I'm a bit more... needy.
SO, he has this lovely FWB.
I have a wonderful bf.
I'm friends with his FWB and working on getting to know her even better.
He's well-acquainted with my bf (we all lived together for 10 years), but wouldn't say they are "friends". Maca's working on resolving the possessiveness issues that arise when facing the reality of my bf. shrug....
I think a lot of people get stuck in looking for "my idea" or "our ideal" and forget that they're dealing with people.
It's possible you could find an amazing single girlfriend and then someday add the perfect man to the group to create the perfect "quad" (or vice versa). But, only if you aren't stuck on ONLY accepting couples as potentials...