New, as of yesterday, Roku Ultra/4800X. The Roku is connected to my Sony STR-DN1080 AVR, which is connected to my LG OLED65C7P TV. I (finally!) got this setup working in all respects except for one, which is admittedly only a nuisance. I.e., I'm getting 4K/60fps video (or at least the Roku thinks I am!), I can turn the two components on or off with the Roku remote's power button, and I can control the audio mute or volume with the Roku remote. Note: the Roku is connected to my AVR rather than to my TV because I have found this necessary to avoid video/audio sync issues. This was the case with both a prior Roku 3 and an Amazon Fire TV stick.
The remaining nuisance is that not long after power-on -- in the neighborhood of 20 sec. or so -- the AVR displays its home menu, interrupting the Roku display completely. I can quash the AVR display with one button press on its remote, with a short black-screen interval before the Roku display returns, but it's a ... nuisance.