I just bought the X900E and at the 3840x2160 res you're running, you can probably only output at 60Hz like mine. You need to drop to 1080p to output at 120Hz.
When I installed the latest drivers for my 1080 Ti, by default the Output color format was set to YCbCr422 at 10 bit color depth (Nvidia Control Panel bottom of Change Resolution page), and HDR in the Windows Display settings was turned on.
Set it instead to YCbCR444, but first change color depth to 8 bit. Like mine, it probably isn't capable of 444 at 10 bit, and could corrupt the driver if you attempt it. At 444 8 bit the color quality, text, clarity, and contrast look way better. It also looks very good in games.
I really don't know why these TVs can't look better at anything other than 444. My cheap $330 Panasonic IPS panel TV never had a problem displaying all of the available color formats in AMD's panel on my 7970. It could be a problem on Nvidia's end as far as the way the TV is recognized. Nvidia seem to be mostly targeting monitors, not TVs.
Another strange thing is my TV says it will not work for HDR unless on HDMI port 2 or 3 (one of which is ARC for TV, which has no HDR content), and only when color format is set to 420, which is the lowest quality.
I really don't know if games that have HDR will run in the default 422 color format Nvidia's driver chooses, especially since my TV says it only supports HDR in 420. There's been a lot of advertising hype around HDR, but it seems to me they haven't got all the compatibility bugs worked out yet. Then again there's very little content for it yet, so that may take some time.
I still have until 1/25 with the option to return this TV for any reason if I wish, but there really aren't any better choices I see under 50" size at this price range. Plus it's unlikely OLED will be made in 50" or less any time soon. My other option was a monitor and TV Tuner card, but that kind of setup is much more hit and miss, and not as good on image quality.