Originally Posted by Tiersten
Its is a bit low but it is still within the USB 2.0 specifications for voltage. Just try it as you can't damage the Kindle assuming your USB charger isn't faulty. If it works then it'll charge. If not then you'll have to find another charger.
The voltage difference is probably OK, but it's not good that the power supply you intend to use provides only half the current of the factory supply. This can lead to damage. Best to find a supply that provides 850 mA, or more.