[Sorry for all the edits. I'm not allowed to post more often that 2bazillian seconds, so the only thing I can do is edit my original post. No one can see the post yet anyway.... *sigh* When I'm done and everything's resolved, I'll add a bit at the end to summarize.]
I'll start a new thread, as I've abused someone else's thread long enough (my apologies.)
So I started out with upgrading my Rpi3 to a 4 and of course getting LE 9.2.1.
Things are running ok in 1080p, but I'm having a couple of problems with the new panel.
1) No sound. I've tried eliminating the Pioneer video receiver -- so the HDMI cable is plugged directly into the TV's HDMI1 input. (more on the receiver in a moment) I've checked Kodi's audio settings and reset them all to default. The tv doesn't have much to say here, just a choice of speaker or optical out or bluetooth: I have it set to internal speakers. The TV seems to work ok at other audio tasks. Not sure what else I can check here, but my next stop is to google troubleshooting sound on Kodi.
Well that turned out to be a simple fix: I used the other HDMI port on the Pi and the sound came back. After shutting down the Pi, I unplugged the power, moved the HDMI cable to the port closest to the power port, then started things up again. *poof* I had sound. I suppose that second port can't do sound. I didn't read the directions. Now to find that S/PDIF cable and see if that will work.
2) 1080p movies play just fine (no sound). I've whitelisted: 3840x2160p 30/29.97/25/24/23.98 and 1080p 60/59.94/50 (curious: there are two 1920x1080p 60 settings... I whitelisted them both.) My test movie (2001 ASO), still has the hesitation issues, but otherwise plays ok. A couple of movies that report 3840x1616 23.976fps are playing, but with weird artifacts. I suspect a mismatch issue that my cpu can't handle. Hm. Actually the second of those is 3840x1608 instead of 1616... meh. I guess those movies are probably just not going to work here. Curiously, a movie that is formatted 3840x2160 23.976 seems to play fine for a while, but then goes black for some reason. Update: I changed the default gui display to 4k and now the movie is playing mostly ok. There's an occasional jerk on long, slow pan shots, but it's a better experience than the 2001 movie. The 2001 movie is also 3840x2160 23.976. I wonder if it might be defective. I just don't have another source to verify.
Anyway... back to the task at hand.
Side note: when I was checking out the 3840x2160 movie that went black, I had the gui set to 1080p. I suspect I was trying to use the OSD to check out the video format when it went black. When I tried to leave the movie running and use the gui, the whole box hung and I had to reset it. That's when I decided to change the gui to 4k and that cleared up the simultaneous gui+movie-in-the-background issues.
Another side note: I couldn't get any 4k resolutions to appear in the gui when connected through my (ancient) Pioneer video receiver. I suspected that the old hardware can't handle 4k and thus always reports the attached panel as 1080p. Eliminating the old hardware fixed that issue. I think I can get the old S/PDIF optical audio cable from the tv to the receiver and do that instead. Later.
Insofar as the 4k issue is concerned: I seem to have proven for myself what chewitt told us over in the other thread: matching video formats work, mismatches not so much, except 1080p->4k.
Anyway, I just wanted to share how things were progressing. I'm off to see what I can do with the sound situation. But 4k is not quite all there, it would seem
I will try putting the hdmi_enable_4kp60=1 back in the config.txt and reboot... maybe that will help with the hesitation issues?