I've also noticed that the Chicago quads are not nearly as abundant as others, and have wondered why. For the Q8s, it may have to do with the double albums. But, one thing that sticks out to me is the fact the SQs do not have gold borders. The Santana 1st album in SQ (which isn't that common to find) also has no gold border, and came out later than the far more common Abraxas, III, and Caravanserai, which do have the gold borders. I'm thinking perhaps the Chicago quads came out later in the game, and the later titles are less common?