A way to solve this problem is to divide the number of cups of grape juice by the number of cups of soda to get a ratio:
[tex]\frac{6}{18}=\frac{1}{3},[/tex]so the recipe calls for 3 cups of soda per cup of grape juice. In other words, we need triple the amount of soda compared to the amount of juice.
Since we are using 4 cups of grape juice, we'll need triple that amount of soda, so we'll have to use 12 cups of soda.