It is now possible to unlock the bootloader (and install TWRP) on the Firestick 4K.
This opens up endless possibilities (root, blocking OTA updates, alternative firmwares/roms, etc).
This is possible thanks to an exploit discovered by a talented coder (with the username: xyz) at the XDA Developers site.
Together with another coder there, k4y0z, they have put together a package for running the exploit. They call it: kamakiri
The forum thread where they discuss this is located here:
Be sure to visit the page there and offer them your thanks (use the Thanks button).
Now I found the instructions there a little confusing, being a Linux newbie, so I have written my own instructions below.
The instructions assume you have a Windows PC/Laptop.
By all means use the guide/s on xda if you wish, or use mine. Whichever you are most comfortable with.
My guide goes a little further than theirs.
While theirs only discusses unlocking the bootloader, I detail how to root the 4K, how to stop Amazon OTA updates and how to completely remove the Amazon Launcher and install an alternative launcher.
I also provide details on installing an alternative onscreen keyboard for the 4K and a file explorer.