So I borrowed a K3 from my friend(about 3-month old) and did some measurement with my i1 spectro, this time using the hi resolution mode so it should be more accurate. I measured each black & white 5 times and used their average as the final reading.
The blacks have a DeltaE of 1.98 which is little high & 0.84 for the white. The K4's black is about 12% darker than the K3 and white is about the same(~1% brighter on the K4). The K4's contrast is about 6% greater than the K3. Below is the approximate representation of their shades in sRGB color space:
Please take the above measurement with a grain of salt. It does not represent the entire K3/K4 product line, just 2 devices I happen to have. I don't have the access(nor the will) to get enough units to make any generalization.
cheers,