Change of volume mixing dry & wet ingredients
This question helps, from it, we learn that 1/2 cup table sugar + 1/2 cup water results in a volume of 3/4 cup.  The second answer claims that 2 cups of sugar + 2 cups of water equals a total volume of 3 and 1/8 cup.  Slightly different from the first response, but in the ball park.  I will assume that the combination of instant coffee and water leads to a similar result.  Therefore...I would calculate:

4 cups water + 4 cups sugar = 6 1/4 cups.

6 1/4 cups + 1 cup instant coffee = 6 3/4 cups (Same assumption as addition of sugar).

6 3/4 cups + 4 cups (1 QT) vodka = 10 3/4 cups.

Total volume = 10 3/4 cups or 2.69 quarts...or a bit more than half a gallon.

Feel free to correct my assumptions or math.