I had problems updating to new firmware 4.1.0 on kindle 4NT.
So I used the kindle 4 main image for firmware 4.0.1 to restore my kindle to brand new state downloaded from http://pastebin.com/Wdw4L7yT
. Then I was able to update to new firmware 4.1.0.
Later when I decided to get into SSH I was not able to use my "fiona5103" password anymore. The only one working was framework/mario that at least allowed me to copy the /etc/shadow file to my computer.
Then I used "John the Ripper" thing which revealed my new root password "fionaadc" and I was able to login as root.
The only one question still not clear is what caused the password change the image from pastebin or the new firmware update. Unfortunately I did not try to SSH before applying the amazon update. It looks like the root password "fionaadc" is stored in the image itself. I opened it with WinImage and it is the same as the one in my kindle.