youtube is compressed video, and at 4k they run like 40mbps, and at 1080p they run like 8mbps. when you view a 4k youtube video on a 1080p monitor your computer is downsizing the 40mbps stream to fit 1080p and the result is more data per pixel and a clearer image. if you view the 1080p stream youre only getting the 8mbps data stream and its not a loss-less stream so its less clear. youtube in 4k is not a loss-less stream when viewed at 4k either, but if you view it on a 4k monitor you will be getting the full 40mbps of content which would be very clear, but at 27inches of viewable area from 2 feet away it would still be hard to distinguish from a 1080p stream with 40mbps (pretty much loss-less)