I upgraded from the 4 to the Air. I wasn't going to, but my mom decided she wanted to upgrade from her iPad 1 and she offered to buy my 4. Since I only had a 16gb and was frustrated with the lack of space, I agreed. But I was still just going to upgrade to a 32gb or 64gb refurb 4. However, once I played with the store model and saw the differences, I decided to try it out. I'm pretty happy with it, even though the upgrade is not extreme from the 4. The biggest thing is the physical size. I really like how light it is in comparison. I read on it a lot and take it whenever I have to wait somewhere, so I really notice the difference in size. And I tried the mini last year, but I'd rather have the larger screen, so the Air is a nice compromise between the mini and the iPad 4. It's a tad bit faster when browsing the web, and opening apps I use, but not that much. However, I have had more crashes from my web browsers than had before, but from what I've read, that's probably an ios7 issue. My iPad 4 is still on ios 6.1.3. I would have probably upgraded to ios7 with that one eventually anyway. As far as the new specs on the air, I guess we'll see what developers can come up with to take advantage of them.
Overall, I could be just fine with the iPad 4, but I enjoy the Air and since half was paid for when I sold my old iPad, and the other half was Christmas/Birthday, I'm happy about keeping it.