4K is generally defined as 3840 x 2160. That means 4K displays have about twice the vertical resolution of 1080p (2,160 vs. 1,080) and about four times the total number of pixels (8,294,400 vs.

When you boil it all down, here's the takeaway: Broadly speaking older and smaller TVs are HD (1080p) while nearly all new TVs are 4K Ultra HD (2160p) which is four times as many pixels as 1080p.