4:2:0 means that only the colour information is subsampled. So while that the colour is at a lower resolution the brightness information is at full resolution. This works because human vision is more sensitive to brightness than colour.
A 4K 4:2:0 image has 3840×2160 luma samples (luma ≈ brightness) and 1920×1080 chroma samples (chroma ≈ colour). So it's still going to have more information than a HD 4:4:4 image which has 1920×1080 pixels of both luma and chroma. What's more, human vision will notice the difference more, so it will look more detailed.
Also it's worth noting that while I've been talking about luma and chroma the information is actually YUV encoded (or more correctly Y,Cb,Cr), where the Y channel is a combination of the red green and blue channels, but it is weighted so that the green channel most strongly influences it (a bit under 60%). This takes into account the fact that human vision is more sensitive to green than red or blue. The U and V channels are the weighted differences between the Y and the Red and Blue channels. This means that more information is recorded for green, and is the reason why video predominantly uses green screen, rather than blue for chroma key.