What kind of TV are you currently using with the receiver? That receiver can handle dolby digital 5.1 and DTS 5.1 and that is the best of it. You won’t get more out of it using an HDMI connection. If you are not converting the audio within the Apple TV 4K, you will be sending PCM signals to the TV. Some will be 2.0, some 5.1 and some 7.1/atmos via Dolby MAT. You are probably getting 2.0 PCM back from the optical cable. Go into the Apple TV 4K audio settings and convert output to dolby digital if you have not already done so.