4k has basically 4 times smaller pixels than a 1080p screen.
So if you were to take one 1080p pixel, you could take 4 4k pixels and put them inside the 1080p pixel.
4k resolution: 3840x2160 (normal 4k) or ("real" [as I like to call it] 4k having a resolution of 4096x2160.
Anyways, when you...