We are also in as quad and feelings run very deep in all directions but that didn't happen overnight. We all started slowly and over time, our comfort zones grew and personal boundaries relaxed. Knowing that all parties want more than just an average relationship has to exist to start with. Ask your husband if this is what he actually wants and where he sees his part of all of that going. He may not feel the same attraction to the other lady and chemistry may not hit the spot for him or he may just fear it more than the rest of you in the quad do currently. The pace at which emotions can grow safely is at the pace of the slowest one of the group and he may feel some pressure from the rest to catch up. Being careful not to push him is important or he may shut down and withdraw, leaving the rest of you in a bad place to be emotionally. Having and maintaining a real quad is not easy and requires really communicating about how you feel, what you feel, and where you see the possibilities taking the group together. Best of luck and maybe try letting the guys talk about it. Sometimes , if the relationship is close in that dimension too, guys are a little more open to that type of discussion and more honest with other guys. Don't let your feelings for the other guy cloud your objectivity. Even if the connection may be really hot, it must have some balance to maintain over the long haul. Best wishes!