We want to find how many liters are 4 gallons. With a quick search we can find that
[tex]1\text{ gallon = 4.54609 liters}[/tex]Thus, if we want to calculate the equivalent amount in liters, we proceed as follows: We multiply the amount we have in gallons by the equivalent rate
[tex]\frac{4.54609\text{ liters}}{1\text{ gallon}}[/tex]So we have
[tex]4\text{ gallons }\cdot\text{ }\frac{4.54609\text{ liters}}{1\text{ gallon}}=18.16\text{ liters}[/tex]so 4 gallons are approximately 18.16 liters.