So I just got my new 4K tv yesterday (LG 55 inch OLED C6P--rated among best 2016 4K tv's). The first thing I did was power up my PS4 and load up Uncharted 4. But to my disappointment, the visuals didn't really look much better. I had an idea to check for a "HDR on" setting in the options, and there actually was; however, it is completely greyed out!!! 

Can anyone tell me why this is?? My understanding was that U4's HDR mode would work as long as you have a 4K tv, regardless of whether or not you had a PS4 Pro. Tried google search but didn't find a definitive answer. 

Did you get the latest firmware?

If it's available yet, this suggests you might have to wait until the 28th
In particular, one alleged conversation with an LG support professional suggested the North American update wouldn’t be published until Dec. 28. Especially for those hoping to get a Pro for the holidays, that could be a pretty big bummer. Such spotty compatibility is precisely why we argued that consumers should wait on Pro until after the seasonal rush.
http://www.idigitaltimes.com/ps4-pro-lg-firmware-update-fix-4khdr-issues-may-be-delayed-most-territories-571385

(You don't have the pro, yet if HDR doesn't work on that, I would think it certainly won't work on normal ps4)

Not sure how it works on your TV. On mine (Sony X800D) I had the option of setting HDR to auto.

And, truth be told, it still wasn't mindblowing. It's more noticeable in Ratchet and Clank than Uncharted 4.

The idea of having to do a TV firmware update makes me ill lol.

I had to do a firmware update on my receiver to unlock full RGB over hdmi back in 2009. Why it would cut the signal to limited RGB in the first place, I don't know. But indeed, day 30+ patch for your new tv to actually work as advertised is a new low.

I think you need to activate it in your ps4 settings first so that your ps4 displays at 4k levels.  Go in settings, sound and screen, video output settings and set your resolution to automatic or highest setting plus activate the HDR.  Hope this helps.

In a way, I think it's cool to add near features to a TV that you already own.

The HDR feature really depends on the game and how a developer utilizes it. On some, the difference is huge, on others (Forza 3 Horizon) it changes the color map to a more unrealistic tone. And on others. like UC4, it's just meh. Gears of War 4 uses it pretty good.

Nevertheless.. I am still waiting for The Rise of Tomb Raider PC HDR Update and Samsung Youtube HDR update.

One more thing I just thought of; on a Vizio that I considered, only one HDMI had 4k support and 60htz (or something like that). On my current TV, all of the HDMIs support 4k but only HDMI 2 and 3 support HDR.

See if your TV has such a limitation. You may just need to use a different port.

In Uncharted 4, the result is not as pronounced as in games like Ratchet & Clank and Final Fantasy XV. But it does look quite a bit more natural. Naughty Dog games tend to have very saturated colors. Yet with HDR, that saturation is toned down to give a more natural, true to life look. Some might like it, some might not. The same happens with The Last of Us.

So far, the game that I think looks the most improved by HDR is Final Fantasy XV. The result is almost night and day. It looks so washed out without HDR on.