The distance... 
Is This Retina? - DPI/PPI Display Calculator
"Remember that there's a separate "body" defining 4K movies at the local movieplex - the Digital Cinema Initiative (DCI). They have "true" 4000 pixel horizontal resolution like the
4096x1716 (2.39:1 aspect), or the very close
3996x2160 (1.85:1) resolutions. Whereas for the smaller screens like computer monitors and TV's, we have the UHD "Ultra High Definition" standard defined as
3840x2160 (16:9 / 1.78:1). So although it's not exactly "4K" horizontally, it's close and I guess "4K" is a better advertising catch-phrase than "2160P". Needless to say 3840x2160 is 2x the linear resolution of 1080P (
1920 x 1080) or 4x the actual number of pixels."