So let the cost of the petrol equal x cents
Then you are paying x-4 cents per liter
and you want to know when it hits 4 percent, so (1-.04)x = .96x
When these two are equal you will be getting 4% off (or think of it as paying 96%), so we set them equal and solve.
x-4 = .96x
x-.96x-4 = 0
.04x-4=0
.04x = 4
x= 100
So when x = 100 cents, then (4%)*100 = 4 cents
So any time the petrol is less than 100 cents per liter, we will get more than 4% off the total price of the petrol.