I demo'd a K4
I recently played with a demo model and didn't care for it. It was too small for my hands. My thumb had to rest halfway across the screen. The page turn buttons were not placed where I could use them. All buttons were too small. I would have to use two hands to operate it.
And it had a dollar store quality to it. Frankly, I think at $79 it is overpriced.
Since it was in demo mode I couldn't use it fully. The page turns were not fast on this particular model. The screen wasn't the best I've seen. I did think the background was slightly darker compared to the K3 next to it but that could be lighting effects. It's possible the unit wasn't functioning properly, though. Other functions seemed strangely tedious.
Checking the other ereaders on display, none seemed to function as intended. Not a good selling feature.