The K4B is the best one with contrast at the moment in the eInk Kindle line but it depends whether you use the keyboard alot or not. It's also super light and tiny. Perfect for one hand reading. It's faster too compare to the K3. Battery Life is about two weeks.
I've own a K3, K4S and now the K4B. I have no negatives on the K4B but I only use the keyboard for searches.
I own a Mighty Bright travel flex light
. 1 AAA battery lasts a very long time and it's weights practically nothing.
I love it but your S.O. in bed might not. My husband wont let me use mine.
The Mighty bright Xtra Flex 2
was too bright and heavy. My husband uses it as his night light.
I own a K3 lighted cover which I think is perfect but sadly he wont let me use it either.
I use a Kindle Fire with Pruss' Screen Dim app to read at night. It works for me. It's about as dim as the PW so I don't get any eye strain. I hear the HD is even better for this.