5k is almost double the pixel count (75%-ish more). It's pretty substantial... also, more to the point with Apple products, I think at the 27" size, 5k is neces

5k is almost double the pixel count (75%-ish more). It's pretty substantial... also, more to the point with Apple products, I think at the 27" size, 5k is necessary for the pixel-doubling high-DPI option to look good. At 4k, everything looks either tiny or huge, depending on whether you have it turned off or on.