I have an 4K LG Monitor which supports HDMI ULTRA HD Deep Colour.  I use the monitor with Windows 10 (Using RX480 8G as a GPU). I also have a ROKU4. I connect both devices using HDMI via a SONY DN860 AV amp. My problem is this. When I watch UHD content via the ROKU 4 it will only work if  HDMI ULTRA HD Deep Colour is turned on which is fine. What that does on the windows 10 display is limit the pixel format to YCbCr 4:2:0 which leaves me with a not so good picture. When I turn off  HDMI ULTRA HD Deep Colour windows lets me use YCbCr 4:4:4 and RGB 4:4:4 (Full RGB). I suspect this is an issue for AMD but any feedback appreciated. By the way when I switch from HDMI ULTRA HD Deep Colour OFF to ON Windows10 detects the change and limits my display to only this pixel format YCbCr 4:2:0
Not too sure how to make the ROKU4 work without HDMI ULTRA HD Deep Colour as that also would give a working solution.