I have both and after the 3.3 update, my battery life has significantly improved on my K3.
The K4 has not been really used since early November and I've only been checking for special offers. I leave WiFi off and I get about 2 weeks charge on it when I do use it for reading.
The K3 hasn't been used in a very long time before 3.3 I got about 3 weeks but now It goes way longer.
It'll be interesting to see how the Kindle Touch compares to the two.
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