One possible issue here is AirPlay. ATV will wake up when someone evokes AirPlay on an iOS device or Mac, and ATV 4 will turn on everything else.
This is an old problem; people used to complain that the light on ATV would shine for no reason. The difference now is that ATV 4 can use HDMI-CEC to turn on TVs and receivers.
I had this happen the first night after I plugged in the ATV 4. I turned it off, came back to the empty den an hour later, and everything was blazing. Turns out my wife had been using her iPad to AirPlay upstairs on the old ATV.
I tested it, and sure enough, invoking AirPlay will turn on the ATV 4, which turns on everything else; I think it will respond even if you arenât trying to AirPlay to the ATV 4 itself, but to some other device.
The only solution, regrettably, is to disable ATV 4âs ability to turn on the other equipment. Look in Settings and turn off HDMI-CEC. My empty den has been quiet ever since.