My LG UF840V is correctly set up as 4K UHD in the one X (no HDR)but the games are displayed at ...

I have the exact same issue on my One X. Tried the Star Trek and Guardians of the Galaxy 2 (Both UHD discs with HDR). Games in HDR (Forza 7) look amazing but UHD movies look awful now.

Regular blurays look fine. My LG OLED C6 switches to HDR mode and displays info saying that it is receiving a 4K signal with HDR and BT rec.2020 so the signal is correct for HDR, but it seems that the Bluray software in the latest update has been messed
up so that it doesn't decode or send HDR image data correctly to the TV as it did before.

you need hdr for 4k output

Actually, you don't.

Uhd and hdr are required for 4k output

You need UHD, in the sense that you need the TV to support 3840 x 2160 resolution, but you don't need HDR enabled to reach 4K. UHD 4K is the amount of screen resolution available measured in pixels, HDR is the ability of the TV or monitor to display more
natural colours.

Note that the Xbox One S can do HDR in games that support such a feature, but regardless of what resolution you set, all games max out at 1080P internally. If the display is 4K, it will remain in 4K, but games will only be upscaled. Only the Xbox One X
can play Enhanced titles in 4K natively.

You should be able to disable HDR, how you disable it depends entirely on your TV. Disabling HDR via the TV settings should not disable 4K screen resolution. Indeed, some Xbox One games do not support HDR, Fallout 4 on the One X can play in 4K for instance,
but it does not support (or indeed have an option for) HDR.

On my Sony Bravia Android based TV, I had to go to the main settings and enable HDR functionality. Even then, I had to set the HDR setting to 'Auto' on the HDMI port I have my console connected to. If you have HDR set to 'On' all of the time, non HDR games
will display the wrong colours, look oversaturated. With 'Auto' enabled, HDR will only operate when it detects an HDR supported game. This may not be a feature of your display however.

What make and model is your TV?

LG HDR PRO UHD 4K is my LED TV 50 inches will the Xbox One S be able to work on this TV?

I assume you are using an Xbox One S/X, which supports 4K/HDR.

Some 4K TV's only display 4K/HDR through one HDMI port. Is this the case with yours? You can also check your TV settings to make sure 4K(UHD)/HDR/HDMI 2.0 is enabled over the HDMI port. Verify that HDR is enabled in your Xbox display settings (set Color
depth to 10-bit, ensure HDR is enabled in the advanced settings).

Since the PS4 Pro seems to be getting HDR color over one of your HDMI ports, what happens if you were to swap it with your Xbox? Do you get your HDR colors back?

you need hdr for 4k output

I don't think you fully understand

Uhd and hdr are required for 4k output

Unless you have an xbox one s not a x

You can't play in 4k

4k is only for.movies or streaming sources which require both uhd and hdr

But.xbox.one x will do 4k gaming