4S Charger Technical Details
Output: 5VDC, 1A (max) Input: 100-240VAC, 50-60Hz, 0.3A (max)
The iPhone's USB power adaptor is slightly more powerful than the Kindle's 4.9V 0.85A,
The problem with wall chargers is:
some work on the kindle and others do not.(my mp3 player charger will not charge my kindle.)
It seems to be something to do with the resistance between the data pins 2 an 3 - some chargers have no connection to these pins and therefore resistance is infinite, the kindle needs to see 1.2 ohms before it will charge. Even Amazon state you need a different charger for different model kindles.
Try it it will not hurt kindle or charger, does the Kindle power led come on? Does the battery symbol show charging but not flickering on / off?