So I tried to make a 10 gallon batch of hefenweizen can I add the other four gallons of water to make it a 10 even though its 6 now or do I need to boil?
It depends what your gravity is. If you dilute the beer that much starting gravity may be way to low. It all depends on where your gravity should be and how much water you need to add to get it there. Other than that you should be fine to add water so long as it's fresh drinking water and not grossly contaminated. However, a full boil is always better just to be sure.