This new display (first image) packs quite a punch, especially compared to the non-Retina iMac (second image). The difference is like pixelated night and seamless day.
We got up-close and personal with the a few of the (over 9 million) pixels in the 4K IPS display. Each is made up of a red, green, and blue strip—vary those and you get a colored pixel.
This 4,096 x 2,304 pixel display packs more than 4.5 times the number of pixels in a standard 21.5" iMac display.