We've "attempted" a couple of quad relationships with existing couples but explored quite a few others. The conclusion WE have come to is that it's just probably not realistic to expect to find 2 other people in an existing relationship that really click with both of us - an vic-versa. But we also acknowledge that the area that we live in is very puritanical and PC, so that may have some impact. Were we in an area that seem to attract more "open" people I suppose the chances might be better. But still I question that.
Our feelings are that if a successful quad ever developed it would be more likely via the coming together of a couple "V"s etc - or something similar.
But good luck to you !