In my experience, relationships with couples are pretty spectacular, though there’s a lot of pitfalls, as you would imagine. Yet they seem to work out really well even so. Especially in bed, as you would imagine.
What seems to be the problem is that the person who is the least into poly wants to keep the relationship only on a quad level becasue it’s emotionally safer, less room for romance that way. Romance seems to scare people a lot, as if they can’t trust romance, as if romance was a madness that was going to sweep them away.
In terms of polyamory, romance can be tamed quite a bit. Since there’s no taboo on loving others, you can have your current relationship and the new one. You don’t have to elope. You just have to convince your mother not to kick you out of the family before taking a practical look at what you’re doing.
But in quads, romance is safer with the larger group because there’s less chance that one couple is going to develop something dangerous. So even if love develops between two members of the quad, it feels safer to explore the love with all four folks together. That affects the way the love will develop.