For what it's worth, I received my Kindle 4 over a week ago and the screen was noticeably less contrast-y than my Kindle 3. The difference was subtle enough that I was willing to live with it, but then I noticed a black spot (stuck pixel?) in the middle of the screen that wouldn't go away. So I sent it back to Amazon and ordered a second one. The screen on the new Kindle 4 is perfect and actually seems superior to my Kindle 3. I haven't taken any pictures, so you'll have to take my word for it. All of this proves to me that e-ink screens vary from device to device, not generation to generation.
Oh, and for the record, I cannot go back to using my Kindle 3 after using the Kindle 4. The build quality, size, simplicity, and feel of the device makes for a much more pleasant experience and it just feels more "high end." And the on screen keyboard is surprisingly not THAT clunky to use... especially since I hardly ever need to use it.
