Apple recently updated its list of Aerial screen savers for Apple TV 4K with ultra high-resolution snippets ported over from the fourth-generation model, as well as all-new footage of Los Angeles ...

It's the resolution. A 1920x1080 monitor can only display that many pixels. A 4k video has enough information to display four times as much: 3840x2160, so four 1080p monitors in a 2x2..