I don't think any one here is right or wrong. I think perception plays a big part. No one can say that they are either. They have to see many K3s and K4s from different batches before it would become fact. Both sides has seen a few different ones and can only tell their personal experience.
Personal perception is what really counts because it's the person who has to use the device. If you're not happy with it than exchange for another one. If you can't find one after you replaced it that works for you then try another reader. What matter is if you are happy or not.
I do think on the text, the rendering has been changed due to being able to see the jagged edges of the text when in bright light. Looks at W,M and Y in a bright light and you will see it's like the K4's anti aliasing has been turned down compared to the K3 where I see no edges at all.
As for the background. It looks grey in cool lighting and in more warmer light it looks tarnish brown at least in my home it does and in the bright light it looks about the same as my K3.
I find the K3 does not change background colors as much under different lighting so something in the eInk process could have changed there too.
I don't think the screen quality itself is inferior in quality but some subtle changes whether it be manufacturer, methods or supplier of materials has changed the clarity and contrast on the K4. Though some devices the changes made are subtle, there are K4s out there where the changes are very apparent like the one I sent back that was so gray it ghost white when going down the book list and didn't look like Pearl text at all!
Another thing I notice now we are in Winter the K4 is more acceptable to tint changes than my K3 when it's cold.
So there are alot factors that make up the tint and contrast differences on the K4.
My K4's background seems to change in different lighting or temps but the text clarity remains the same no matter what it still has rendering isses.
I do think a firmware change would be wonderful if they could do something like the Nook Touch did that would be great!
The darker the text the better the background will look. I change my K3's font to a bolder font and the background became near white before it was a medium creme color.
So my conclusion is that my K4 is more susceptible to lighting and temps than my K3 when it comes to the background tint and lightness. I have been able using this theory get rid of the greyness on the K4 by changing the light to something warmer. I doubt it would work on the one I sent back.
Now if they could just fix the text issue I would be very happy.