Ok, I think i've found out why can people see the difference between the K3 and K4 display.
As you can read on Tom's Hardware site, the K4 screen seems "lower quality". At first I have to say, I have my K4 now, and I can make side by side comparison with my brothers K3: there is no difference. They are the same. (Maybe pearl screens may vary individually, no matter wich device you have) But, as we can see there is a modified screen refresh/page turning method. I can see that: as more as I turn a page, text becomes as more "poorer" or somewhat gives less contrast, seems not as detailed. And that's why it needs a full refresh after turning 6 pages. With the latest update, you can turn the page refresh type on/off.