In your drop checker, you wish to have water what you know the dkh of. Therefore, when you put the pH reagent in it, you know what the colours will represent. If you don't use a 4dkh water in the drop checker, then the standard colours of blue - green - yellow will not provide an accurate reading, as the pH of the drop checker water will be affected by the other items that are within the water.
but if you check th kh in your tank using a kh test kit then you already know the kh of your water in your drop checker,i was only asking because the jbl drop checker comes with a colour chart with different kh values see below
it also has different ph values aswell,so if i use 4 dkh in this checker to get the lime green colour that i hear so much about it will mean a ph of around 6.6- 6.8 but that will be to low for the nerite snails i keep,please tell me if i'm seeing the whole thing wrong.
Don't forget that the pH you are measuring the DC with is literally that, the pH in the DC, the pH and KH in the tank could be whatever you want them to be!
EDIT - the DC is just a useful way of keeping an eye on the CO2 level in the tank. We use 4dKH water specifically because that is just the KH that happens to give nice easy to read colour changes, i.e. blue = low CO2, green = CO2 good, yellow = CO2 high. If we used say 6dKH or 2dKHwater these colour changes would be different and not as easy to read, that's all it is