When my FireTV Box 2nd generation died I recently replaced it with the Firestick 4K. But for the life of me could not find a version of Clean Master that would work on it (or at least well enough without jumping through a lot of hoops). I have, however, found an alternative that works almost as well as Clean Master: DB TV Assistant.
You will need Filelinked on your device. If you don’t have it here’s the link to put into Downloader: http://get.filelinked.com
Once Filelinked is installed enter this code into Filelinked: 11111111 (all one’s). Once on the page scroll down until you see Clean Master in the list (Clean Master info states to use DB TV Assistant for the Firestick 4K). Right below Clean Master in the list is DB TV Assistant. DB TV Assistant works for ALL devices, including the Firestick 4K.
I just downloaded, installed and tried it. It does the job. First run I had 89% free memory. After running DB I had 99%. So it works for me.
Hope this helps those who are looking for a utility like Clean Master but are having trouble finding something that works.