There is really only one way to determine which has a better screen for you--see it in person. I've tried photographing at least half a dozen eInk screens over the last couple of years and the photos never tell the whole story and often don't tell it accurately. I've read several articles from others stating they have the same poor results in photographing eInk screens. It makes sense because of the reflectivity of the screens and the ever changing lighting.

I always suggest to people to judge the screens in their own comfort zone, that is, where you normally read. Comparing screens in a retail store will probably yield different results due to the type and brightness of the lights than if you compare them in your recliner next to the fireplace!

And of course everyone's eyes are different. What works best for me might not work for others. All I can say is that under my set of reading conditions the over all screen contrast is better on the K4. If I tilt the readers a certain way, then the K3 is better, tilt it another way and the K4 comes out on top. But over all, I find the K4 screen better for my eyes and my lighting.