I don't have the exact numbers in front of me at the moment, but a 27" 4K monitor will not match the pixel pitch of every other Mac -- screen elements appear larger when both are set at the same scaled resolution.
Yeah, to match the dot pitch apple is designing for, the 4K monitor would have to be more like 22 inches instead of 27. We know this since the 4K iMac is a 21.5 inch screen.