I really don't think the eInk screen is any different on the K3 and the K4. They are both Pearl 6" screens. I do think that either Amazon used a different top layer over the K3 screen and/or they tweaked the firmware to make the K4 screen work better. I've read that the eInk screens have much more capabilities than is utilized by current readers, so perhaps they tweaked the software to take better advantage of some of those capabilities.