Simple syrup end result
6 TBS (3 ounces by volume) of granulated sugar, mixed with 4 liquid ounces of water (1/2 cup US), brought to a hard boil in the microwave yielded just over 6 liquid ounces of syrup. (A drop of color added for readability)



I would suspect that it didn't reach 7 ounces because of the air included in the volumetric measurement of granulated sugar.

So 4 cups of water plus 3 cups of granulated sugar would yield just over 6 cups of syrup if not allowed to reduce on the stove.