I am typing this thing on an iPad 2 as we speak, I love this little machine. I will keep on using it until the battery turns to shit, then I will buy the 64 gig model of the newest iPad. This iPad 2 will get a second life as my picture frame sometime in the future.
It makes sense in a way, because iPad 2 parts were probably getting expensive since they are so old. Also, the iPad 2's sole purpose was to be the gimped machine to persuade people to buy the newest one since it is "$100 more." The gimping was the old processor and the non-Retina display. For the iPad 4 compared to the iPad Air (5) is the old processor and the heavy weight. Expect the 4 to be there for the next 2-3 years, the same way the iPad 2 was.
So don't expect the iPad 4 to be replaced for a long while, iOS 8 will likely be the last update for the iPad 2. You had a good run buddy!!