It is your battery.
And on the K4 - the battery is glued in.
You are correct -
the K4 firmware **does** check the battery make/model.
So using an external battery is out.
Also it is possible the flash chip has been damaged.
See:
https://www.mobileread.com/forums/sho...49&postcount=3
The good news:
The new KT2 is the same price the K4 was sold at.