It actually does matter...if you measure out water first, then mix in powder, you will see that the amount in the end is actually more. So if you measure out 4 ounces, then add two scoops of formula, you end up with a little more than 4 ounces in the bottle.
Conversely, if you add the formula first, then add water up to the 4-ounce mark, you are adding less water.
I do it the first way because that seems to be what the instructions say to do.
At night, we keep dry bottles with two scoops of power in them in LO's room and add water when he needs a bottle (saving a trip to the kitchen), so I add water until it is a little over the 4-ounce mark to try to approximate what happens when you mix is correctly.