The statement of Netflix rules hasn't changed, but either the Roku firmware or the Netflix app has, in the last few months, to prevent 4k streaming. I'm not particularly interested in who's responsible. I'd love it if this got the problem fixed. But more likely I'm just informing anyone else in my situation so they don't waste more time than necessary trying to fix the problem or better yet doesn't waste the money in the first place. To those people, it is possible to find a relatively inexpensive and tiny PC that can get the job (4kUHD/Atmos) done.
If you have the option of connecting the Roku directly to the TV and getting audio from an optical cable to the TV (can't tell for sure, but doubting that the receiver supports ARC), you should be able to get 4k. You'll be stuck with DD sound (again can't tell for sure if the receiver decodes TrueHD, so maybe not losing anything). Depends which matters most to you.
Yes, shortly after posting I did locate the user guide for the receiver and discovered that it is not 4k.
The fact that I'm dealing with an inherited Control4 system that is over 12 years old with no documentation is not making it easy to figure out. I didn't buy a 75" TV to watch at 1080p. So it looks like a new receiver is in order. Any suggestions? I'm totally out of the loop.
If you can tolerate DD audio and the Roku was a new acquisition/present that you could trade in on a (4k) Roku stick, that's still the simplest route to 4k video to avoid tearing up your pea patch. There are also HDMI audio extractors that would allow the video to bypass your receiver, and just have it handle the audio. To get the idea, see 10 Best HDMI Audio Extractors of 2022 - Audiosolace .
If you want to get the new receiver (or use an audio extractor), keep in mind that your HDMI cable(s) from the receiver to the TV may be limited to 1080p also. Hopefully they're easy to replace. For receiver choice, agree that avsforum is a good source of information.