Offical YouTube app available to install now for Amazon FireTV and Firesticks
Does not get automatically installed with FireOS update, you need to go Get it.
I had it just installed on my 4K Firestick today. Register the device and signed onto Google account and working great.
EDIT: Has a really annoying bug - voice navigation is on all the time and I'm not able to turn if off. Hope it gets fixed soon.