For this purpose close enough is close enough. Figure a gallon is four liters and you'll be close enough. It isn't of course - four liters will be about a cup more than a gallon, but it's more than close enough for diluting wetting agent.*

I'm thinking of re-calibrating my car speedometer in furlongs per fortnight...

*More precisely, 1 US gallon = 3.78541178 liters. But like I said, close enough.