I had a look at the demo models in Waterstones today. Putting the K4B and the PW (with the light at it's lowest setting) side by side the screen on the K4 looked very slightly lighter, and the fonts ever so slightly darker, but the fonts on the PW looked better, more smooth and solid.
Turning the light up to even just 5 or 6 made the PW screen look a whole lot better, and didn't do anything to the fonts. It was only with the light turned up to 15 or more that it started to lighten the text.
Obviously being in a shop I couldn't look at the screen in the dark, but I couldn't see a hint of colour on the screen, and the lighting was fairly even along the bottom, better looking than most of the photos I've seen posted - even the good ones.
On the basis of the two demo models in that one shop, I'm pretty tempted by the PW now!
Edited to add: the only thing I didn't like was the text justification, it was set at it's widest but seemed a bit too far in from the left of the screen?