I have to respectfully disagree with a lot of the comments made in this thread. I find the K4 quality to be much better than the K3. I've owned several K3's and each exhibited a certain amount of "flexibility", so to speak, when squeezed. The design of the K3 also uses a layer of double-sided tape to adhere the bezel to the screen. In many cases, this delaminates and the bezel no longer adheres to the screen.
The K4 is very solid and the metal back provides to an overall feel of of quality not found in the K3. Bezos himself talks about the K4 having the best build quality. I know there can be a little variation in screens, but the perceived darkness is because of the contrast between the bezel and the screen. Ever look at a white K3? The screen looks dark, for example.
I've been impressed with the design and have no issues reading with reading with one hand and turning the buttons. My hands skew to the larger end of the spectrum. Compared to the Sony T-1, the K4 quality is better. As a side note, my daughter owns a Nook Simple Touch, which seems to have a quality build. No issues with the buttons on that design either.