Thanks for all the responses, everyone.
I finally had some time to troubleshoot, following some of your suggestions and looking at the manuals and onscreen help for the various devices involved. I’m pleased to report that my Roku is finally acknowledging and using my 4K setup.
—I upgraded all the HDMI cables for my entire setup to ones that accommodate eARC.
—I made sure the video setting on my AV receiver was set to 4K Enhanced.
—Then, I found one more setting on the TV itself. It was buried in a somewhat obscure, nonintuitive location. But I found it. Essentially, this setting enabled connected devices to display in 4K. And, by default, it was turned off. I turned it on for the first time...
—then tried the video settings in the Roku again...
—and, just like that, the Roku started to display in 4K with no more errors.
The other thing is that the 4K toggle on the TV is applied on a port by port basis. So, the particular feature can be turned on for one device, but left off for others.
So, the resolution is, at last, resolved. Now I just need to calibrate the set and fine tune the color...
Then, I’ll try to avoid touching the settings again, lol.
So many variables.