How the test solution changes has nothing to do with your water's KH. The 4dKH and pH indicator solution are separated from your tank's water by air. Regardless of your tank's KH, the air buffer will increase in CO2 concentration as your tank's CO2 concentration increases, and will then cause the CO2 concentration of the indicator solution to increase. As the CO2 concentration of the solution increases, the color of the solution changes from blue to green, then yellow.
At 4dKH, the solution changes to forest green at around 30 ppm. This is why we use 4dKH - 30 ppm is a good base concentration to shoot for. If you were to increase or decrease the dKH of the solution, you'd get a green color at higher or lower concentrations of CO2. If you had a 2dKH tank or a 10dKH tank, 30 ppm CO2 would still result in the same green color. Though it's possible one would have a faster reaction time than the other, given the same drop checker and solution. I'm not sure. Some people do use 3dKH or 5dKH (not many, but some) if they want a different CO2 target concentration.