The colour gamut and contrast are better with 4K that 1080p, but both the content and your monitor/TV have to be good quality and support both 4K and HDR. The situation isn't helped by there being six different types of HDR (that I know of): Dolby Vision, Dolby Vision IQ, HDR10, HDR10+, HLG and Technicolor. HDR10 is the most commonly implemented. In addition, the display has to be correctly calibrated to achieve the best image and hardly anyone is willing to pay for an expert to do that - I just fudge it myself until I'm satisfied. I see most improvement due to the HDR and not the resolution, possibly because even though I'm short-sighted, my colour perception is very good.