So I have a 65" OLED, obviously one of the best there is, so take that with a grain of salt - but I know people who have TCL 4k HDR 55" and 65" displays and have the same opinion as me.
1) For games (If you have a PS Pro / Xbox One X - it is a HUGE difference. 4k (Whether checkerboard or dyanmic (Don't see a lot of native) while HDR is enabled is absolutely stunning. God of War, Uncharted 4, Last of us Remake, Lost Legacy, and others, are absolutely fantastic and definitely worth it in my opinion if you have those gaming devices.
2) For movies, I now find I want to view all content in lossless audio with 4k kHDR whanever I get the chance (Even if it is only HDR10). Everything pops and is crystal clear. With that said, it depends on your view distance from your display and how large your display is on how much you will notice 4k.
3) Some Caveats: If you have a home theater set up with speakers (5.1, 7.1, 5.1.2, 5.1.4 etc) and are driving your devices through a receiver, it gets tricky and can get quite expensive. Reason being is that to pass 4k/HDR your devices and receiver must have HDMI ports that are HDCP 2.2 Compliant or it will not pass. A way around this is if your receiver and display are capable or Arc, and you plug a device directly into your display and use the Audio Return Channel to get sound back through your audio system.
For my upgrade, I had to get the display, and also the new receiver. (Also some new cables with the bandwidth required to pass 4k HDR). (I then also upgraded to an Xbox one X (Mainly for the 4k / HDR player) and a PS Pro.
All in all, best investment I have ever made currently. Next up I am looking at Atmos (Still). I kick things around for 6 months... lol.