Yes Quads do seem rare indeed. I find myself searching very hard on this forum for others in the same situation as my own.
I am part of a closed Quad. Our group is somewhat unique I think. Both couples are originally monogamous with no one having any experience in polyamory, open relationships, etc... We both have families and keep this a complete secret. Both couples have been best of friends for years before anything happened. Oh ya, we also are neighbours. The guys are straight, the girls... straight-ish :-)
We are about a year into it now. LOTS of ups and downs. The ups are amazing and the downs are sometimes horrible. The closeness and love we feel for each other has helped us through the tough times. Jealously, possession, insecurity... bla, bla... the forums here are packed full of the same problems for good reason. This type of relationship requires a ton of work, patience and above all communication, even when its the last thing you wish to do.
One really important issue that we continue to work on is the speed of which each member wants to go forward. We found the hard way that its best to move as fast as the slowest person(s). That way everyone stays happy with far less problems. It can be tough to hold back but for us it is the only way it works.
Another communication trick that works well is the 4-way group text. No more he said - she said. Thoughts and feelings are always out in the open and always clear to everyone. Also, no one speaks for anyone else, and everyone has a responsibility to speak up or shut up. Sounds harsh but its sucks to have to worry about what other people may be thinking... it makes you nuts and it simply creates problems ;-)
Our Quad is on an upswing atm... we have had lots of "fun" together lately and there is talk of moving forward allowing secondary relationships to build on their own. Like you we are all very happy but always cautious at the same time. My marriage has never been better, communication and intimacy is greatly enhanced. I thought my marriage was awesome before but now... incredible!
The togetherness, love and support everyone gets in a relationship like this is hard to explain. The funny thing is that some members of our group refuse to label us. There is a fear using the terms like Polyamory, BF, GF, etc... so far its not used (accept for me :-)) Yes, I get in trouble for that.
Our greatest fear is making a mistake... getting caught and eventually having to come out. It's probably always seems worse in theory vs practice but holy cow I can't imagine how difficult that would be to explain. Its sad really but its also no one business either.
Glad you are having fun... we are too!