Well it seems tonight's adding books and reading did it in. I got the low battery message. My K4 is being charge for the first time. 21 days with heavy use, adding books, multiple reboots, and WiFi on is not bad at all! I get about the same with my K3. So I am very pleased.
I suppose it would go longer with WiFi off.
So while it's been charging I've read on my K3. I really forgotten how heavy it is! I do like that I get more words per page though and I can turn the page one handed. However I miss my K4 terribly already but I do now see how having two Kindles is worth it when one needs charging.
