I had both the K3 and K4 for a several months. Screens are the same size and viewing contrast was about the same. The K4 is a nice reader especially if you like the smaller footprint. In practice I found I used the K3 way more than the K4 as the K4 was more difficult to enter text into. I didn't like the silver-grey finish of the K4, preferring the charcoal-grey of the K3 which doesn't reflect light as much. The glare from the K4 body, not screen, while reading was enough to make my eyes strain a bit too much.