How to connect Apple TV to a 4k receiver

The best way to connect is with an HDMI Cable. By the way, make sure to change your volume controls on your Apple TV to change on the receiver, and not the tv.

For use with (modern) AV receivers, I would suggest to connect as follows:

Apple TV →[HDMI]→ AV receiver ⇄[HDMI (e)ARC]⇄ TV

Set up Dolby Atmos or surround sound on Apple TV with a sound bar or AV receiver - Apple Support

Play audio in Dolby Atmos or surround sound on your Apple TV - Apple Support

Using the HDMI 2.1 cable, 

connect the HDMI IN (ARC or eARC) port on the TV and the HDMI OUT (ARC or eARC) port on the audio device.