I am assuming Michael's interpretation is correct, that you want to find how much gas is used in 4 days knowing how much is used in 5 days. Set up a simple proportion and solve it:
3/4 tank → 5 days
x tank → 4 days
x = 4 * (3/4) / 5 = 3/5 of a tank
In 4 days you use 3/5 of a tank.