Reposting from another thread:
On my Yamaha 2060 you have to go into a special menu to select 4K hdmi mode for the hdmi port for 4K (defaults at 2K) and another menu to turn on arc and hdmi control. Then at that point it will flicker until you use a certified cable for hdmi 2.0 on all connections. Sony UHD Blu-ray and Apple 4K TV and Samsung 4K TV play nice for me but they all support HDMI 2.0a spec so nothing complicated.
And reposting how to change hdmi mode:
Advanced setup menu you get to by powering off then hold straight button on the receiver while powering on. Then use “program” button to scroll through the menu and “straight” button to change a setting.