If your tap water is 4dkh you can just use it straight.
That said, before you count on it being 4dkh, I would take a sample of the tap in a large cup and leave it out for 24 hours and then test it again. There is a relationship between the amount of CO2 in the water and the KH value and the amount of CO2 in the water might be more or less than normal right out of the tap. 24 hours should be plenty enough time for the CO2 level to stabilize and that will give you a good idea if your water is actually 4dkh.
You'll also want to check the tap water parameters a few times a year if you live somewhere where the temperature changes a lot over the seasons, especially if you have well water or water that originates from wells.