Thanks for waiting!
What I found confirms my experience from earlier.
PS4 Resolution = 2160p - RGB (manually set)
RGB = Automatic
HDMI Black Level = Automatic AND Low
-> Picture looks proper. Dark spaces and good contrast with discernible detail.
BUT, at:
PS4 Resolution = 2160p - RGB (manually set)
RGB = Automatic
HDMI Black Level = Normal
-> The picture gets washed out. Way too pale and grey to be natural.
This tells me that even when the RGB resolution is forced (2160p - RGB), the PS4 does gameplay
in a way that matches the TV's Low setting.
But when I do:
PS4 Resolution = 2160p - RGB (manually set)
RGB = Full
HDMI Black Level = Auto AND Low
-> This is a mismatch. Blacks are crushed. A cave opening in Tomb Raider that's dark but still shows subtle details of rock outcroppings and the like becomes just pitch-black -- no detail whatsoever.
Interestingly, it seems the TV's Auto setting is not distinguishing when the PS4 Pro setting changes. No matter what, Auto is always the same effect as Low, whether that means a match (because the PS4 is in Auto/Limited) or a mismatch (when PS4 is in Full)
Finally,
PS4 Resolution = 2160p - RGB (manually set)
RGB = Full
HDMI Black Level = Normal
-> This is a great picture and near-identical to Auto/Auto | Auto/Low
TL;DR - PS4 Full plus TV Normal is a match and looks near-identical to PS4 Auto plus TV Auto, which is also a match. However, my 55" KS8000's "Auto" setting is just a synonym for "Low" -- it always matched whatever "Low's" effect on the picture was (regardless of whether I was outputting Full or Limited), so I'm not even sure HBL Auto is working like how we expect.
Indeed, switching between Limited and Auto on the PS4 caused nothing to change. No momentary screen cut-out, no color range, nada. But switching from Auto to Full caused a 1-2 second blackout.
Unless I'm missing something, I've concluded that PS4 RGB "Auto" outputs a limited range of 16-235, which both TV Auto and TV Low match. That's why Auto/Auto is a fine setting that will look great for all kinds of content. But upon forcing PS4 RGB to Full, Normal is the setting that matches.
If you want to play your games with the Full RGB range of 0-255, I believe Full/Normal is the only way to do that.