Hint: the K4 is great. It's unbelievably light, and provided you don't need to make annotations, pretty easy to navigate, especially if you've used a K2 or a K3.
(FWIW, it's the one sitting in my bag everywhere I go, not my PW2, so, thanks @GM for sending me one ;p).
But to answer your original question, I personally think the concept of a backup reader *device* is a bit over the top. If you happen to have a smartphone: presto, backup reader!
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