That means you can flash the update using TWRP or ClockworkMod to upgrade from Kindle Fire 6.3 without losing root access or your custom recovery. The software is pre-rooted and installs busybox ...

Fire OS 4 will be pre-loaded on Amazon’s latest tablets, and will come to existing Kindle Fire HDX tablets at an unspecified date. Amazon’s Fire phone will get the update early next year.