I am trying to understand the situation you are trying to describe, but I need more information.
First of all, why do you have 2 Apple TV’s connected to the receiver?
Every television out there has 2 or more HDMI inputs.
Connect the 4th gen to the HDMI port which has the ARC support.
Then connect an optical cable from your TV to the receiver.
The HDMI port with ARC support will relay the sound from the 4th gen Apple TV to your receiver via the TV.
Then you can either use the 3rd gen in another room or resell it to someone else.
If you have any questions please
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Edit: here is some extra information about HDMI ARC
https://www.digitaltrends.com/home-t...ed-works-care/