I switched from a K3 to a K4 last year as well. My K4 screen was not as good as the K3, but I liked this little reader so much I kept it and gave the keyboard to my Mom (who still had a K2). I did try one replacement, but it was not an improvement.
GlassX - I don't miss the keyboard or any of the other features at all! Typing is a pain, but I do it so seldom it is OK. I you type often, it could be a deal breaker. I don't feel like I downgraded at all.
The Paperwhite and the light made me think I could try going without the physical buttons for the better contrast the light gave. My husband got one, and after reading on it yesterday, I am still not convinced I would be happy with the touch.
After reading this thread I am thinking about trying a new K4. I know that e-ink is very inconsistent, but it sounds like maybe the have weeded out all the bad ones. Thanks for the reports!