I bit the bullet during a sale and did the 4K upgrade with 75" Sony X90F and Sony UBP-X800. Had to build a pine cabinet for it too which was fun.
Anyway, after a couple of weeks of monkeying with the TV settings, I got it looking the way I want it and I watched Christine (1983) on 4K last night (NZers/Australians: I got it for $21.50 delivered from JB-Hifi - arrived just in time for Halloween too).
First of all, the 4K/HDR picture is awesome. Way more detail and graduations from dark to light. Well worth the upgrade.
The 4K has a new Dolby Atmos surround mix (along with what is presumably the old DTS-MA 5.1) and although I just have a 5.1 setup, I selected the Dolby and it sounds incredible. Fidelity is great and there's way more happening in the surrounds than there was on previous releases. There's panning to the rears when Christine moves off-camera, engine revs from the rears when she's behind you and when Arnie and Harry Dean Stanton are yapping in the parking lot, there are people playing tennis behind you. At least, I think it was then. Definitely tennis at some point (unless I was doing a Blow Up David Hemmings).
The rears also add an awesome sense of large space in quiet scenes. At Darnell's when there's not much going on, there must be something coming from the rears (I didn't get up to check with my ear against the speaker) as it sounds like a large, echoey space except without any obvious echoes. I've not heard that before in a movie but it's really effective.
Carpy's synth score sounds great too. Man, what a movie. I got a bunch of other 4Ks on the ways from the UKs too (I'd happily by them in NZ but stocking popular movies seems too hard for NZ retailers) so I'm looking forward to spinning them up.
Just a FYI if anyone is thinking about the 4K upgrade, I was recommended a Panasonic cable with "4K" on the packet but when I got it home, the fine print stated it was 10Gbps but the Sony UBP-X800 manual stated 18Gbps is needed to pipe the entire disc content to your telly so I had to hunt out a computer store to find one. None of the salespeople in 2 stores seemed to know about the 18Gbps requirement. Oh well, it's all plugged in now.