Yes the 4th Gen was quicker than the 3rd Gen, but I don't think that's the main reason people were buying them, infact its things more like improved cameras and such that are bigger advantages for most.
I thought one of the key reasons they released the 4th Gen in spring was simply because of the new Lightning connector. Given that the iPhone had it and now they were selling all new accessories that used it, especially as people were not having to buy such things to replace the old dock connector, it mean't they had to get an iPad out there with it.
We are somewhat in the same situation again though given that the iPhone now has Touch ID, which was one of its biggest new features, yet the new iPad doesn't have that, so it again seems like a strange omission as its the kind of thing you would expect to have in there. So I don't think its impossible that they might release a new one in less than a year. Obviously though compared to the Lightning connector, the addition of Touch ID doesn't bring in extra revenue from further sales.
