For my usage I can't imagine any change that they could make to the K3 that would make a K4 a "must have" upgrade. The screen has reached a level of readability that is more than good enough, the page turns are fast enough (I don't even notice them anymore), the size/shape/weight are great, the battery life is phenomenal, etc... It's very rare that I feel that way about an electronic gadget, so that's saying a lot. It probably has to do with the fact that it's meant to (and designed to) disappear in your hands, to become something that you don't actively think about while you're using it.
I guess that's a long way around to say that I can't see any reason to wait.