Hi Hostar, thank you very much for your explanation! A lot clearer than what I had found up to now.
As I'm not a Unix/Linux guru, I will not try that, I'm too scared of bricking my Kindle. There are also a couple of steps in the SSH procedure that I don't really understand (such as installing the driver on Windows - basically I would install a network card driver for the Kindle connected via USB? I have a hard time getting it, let alone doing it).
Thanks for the recap! And thanks everyone for their efforts freeing up the K4/5.