The comments aren't working at Walk Like a Sabermetrician so I'll leave the answer to the trivia question there in my extended entry, in case you want to try it before it's revealed.

The four teams are the first four expansion teams of the modern era. The Rangers were originally the new Washington Senators, after the original Senators moved to Minnesota in 1961. The Senators/Rangers and Angels came into existence in 1961, the Astros and Mets in 1962.