yep...in my mind...As somebody that enjoys Belgians, I am often confused between the BJCP Belgian Blond Ale, Belgian Golden Strong Ale, and Tripel. There is a lot of overlap between those styles and many commercial examples don't cleanly fit into the descriptions.
But I do look at a Single being the smaller sibling of a Tripel (though the Single style comes in at a higher IBU range than a Tripel), and a Dubbel being the smaller sibling of a Quad (aka Belgian Dark Strong Ale).
1&3 are related...lighter flavored, regular or higher ABV
2&4 are similar...darker and bolder flavor, regular or high ABV
evens and odds...