The best way to make a 4 dKH standard solution is to start with a known certified standard KH solution. You can buy such solutions from chemistry supply places. The dilute it with distilled or DI water to get the 4 dKH you want. I think the formula for how much distilled water to add is:
volume of distilled water/ volume of known dKH water = (4 + known dKH)/ 4