Thank you very much for your work !
Thanks to the tuto created by Kubbur and Danar, I managed to change the screensavers on my kindle 4 NT, without any knowledge of what the hell, I was doing.
However, I was blocked untill I found this indication on how to get the "Unidentified Network" linked to my Kindle in the "Network and Sharing Center" :
>> Instructions for USB Tethering on Vista and Windows 7
Maybe, it could help another beginner, to get the root access via USB Networking.
One more time, thanks a lot.
Good luck for the full jail break.
Merry christmas !