The iPad 4 was a soft launch
The iPad 3 came out last March with a big press event and lots of publicity about its 'retina' screen.
The iPad 4 came out eight months later in November, having been mentioned almost in passing at the iPhone launch with "oh, and we've added the new connector to the iPad too".
The iPad Air has had the big keynote treatment again.
So, yeah, 4 versus Air is technically accurate if you're asking about Apple's year-on-year health, but 3 versus Air would be more realistic, I think. Otherwise this is more like trying to determine how much Apple benefits from its usual method of product introduction.