Hi I'm also having trouble playing this 4K HVEC video on my RPi-4B-8GB.
I'm overclocking and increasing gpu_mem. Here is my config.txt.
unless it is HDR, that is not fully supported yet.
The video format is HDR10, so maybe that's the main problem. Although, I've experience flickering when moving around Kodi's menu and also with the Megadrive emulator (but maybe that's an unrrelated problem).
Btw, I also have an LG Tv and just set the HDMI Ultra HD Deep Color option and add hdmi_enable_4kp60=1 in my config.txt