There are standard 4K LED LCD TVs and 4K OLED TVs, All 4K TVs double the number of pixels across that of a standard HDTV, so 1080p becomes 2160p –giving you 4K resolution (4 times the Full HD resolution).
The OLED TVs have a different internal structure than that of a standard LCD -making them super thin and very lightweight.
Also the picture quality is stunning, with Infinite Contrast that ranges from blazing whites to the darkest blacks. Have superior viewing angles that gives you consistent colour at wide view angles.
Both OLED and the standard 4K TVs offer great picture quality. Ultimately it depends on the quality of the media which is being played back. Otherwise you have to keep watching it in the Demo mode.