I believe that I've found an easy to reproduce method to get an ssh root shell on the kindle 4 (no touch) in 3 easy steps:
1. Reboot into diag mode
2. Enable usbnetworking and configure the computer side of the link to 192.168.15.1 (or anything in that subnet)
3. ssh to 192.168.15.244 using "root" as login and "mario" as password.
A few items worth mentioning:
* My kindle answers "yes" to the question "is this a prototype board" in some init scripts. I'm lucky I guess. This might mean that the above instructions won't work on all the other kindles. For example yifanlu's kindle touch doesn't have the dropbear binary in the diag image so it doesn't work.
* I have a working serial port on the kindle which helped find the password for the diag image.
* usbnetworking can be found in the following menu in the diag menu: Misc individual diagnostics > Utilities > Enable USBnet.
* The "normal" 4.0 image used init.d while the diag image uses upstart.
I'll be putting information and pics on http://dionoea.chewa.net/kindle/