Hey, I wish I’d done more homework before purchasing one thing or the other a couple of years ago. I’m still learning this stuff. It's no little thing to go from a system five or more years old to new stuff anymore. HDMI and HDCP(The Devil) specs have made things ridiculous. It doesn’t help when there is no standard among brands, either. Honestly, if anybody with an older system wants to jump head first into the 4K HDR world, I’d say scrap everything but the speakers and make sure the components are of the same HDMI and HDCP specs and get premium certified/ultra certified HDMI cables.
Oh, and while on the subject, set the Apple TV 4K to 4K SDR Format. Setting it to HDR converts all SDR material, including the menus, to a fake HDR. So Format: 4K SDR, HDMI Output: YCbCr, Chroma 4:4:4, Match Content: Match Dynamic Range ON and Match Frame Rate ON. These settings will keep SDR content in SDR and HDR content in HDR and everything will playback in it’s native frame rate which is 24fps for movie material. Then, if available, use a Smart Phone for Color Balance in the Calibration setting. It actually works and improves picture quality a bit.
Audio should be set to Auto with Dolby Atmos ON. Anything NOT in Dolby Atmos will come over as PCM with this setting and Atmos tracks will display as Atmos/PCM. There can be a delay in the audio output when it switches to Atmos.
Many don’t like it, but HDMI-CEC works beautifully with the new Apple TV 4K remote control so give it a try.