Ok I just picked up the Apple 4K TV..here's my setup:
Connect LG C9 HDMI 2 (arc) to AVR Marantz MAIN OUT
Disable Arc on LG
Connect Apple TV HDMI to HDMI Media Player Input on AVR
Direct Connect Ethernet cable from modem to Apple TV
Setup an Activity Macro with Harmony Remote to activate LG, Marantz, Apple TV aka "Watch Apple TV"
I will report back once I play around.....Cheers guys.