Yesterday I had several anxious hours after trying Librerator on my K4. It worked, and I've tested several epub, pdf and chm files, but when I've pressed Exit, it has exited and in 3-4 seconds launched again. After 3 tries of exiting and self-relaunching, my kindle has started to reboot, but Librerator has started again, and new reboot has followed. After 2 or 3 reboot cycles I've seen "Your Kindle needs repair" message and the word 'framework" in the bottom of kindle's screen. Brick. Even USB disc was not seen by PC.
The word 'framework" in the bottom of kindle's screen definitely pointed at some problems in /var/local partition. Due to some happy coincidence, yesterday I've copied the entire /var/local to tarball to find and back up cookies (without ssh). So, after several attempts I've launched usbnet mode (via fastboot), clean /var/local and restore it from tarball.
Now my K4 works as before, and I've got some new experience. But I wouldn't recommend anybody to install Librerator on K4.