Why? You get no more or no less gas mileage with a full tank as compared to a 1/2 tank. Half filling it 4 times is the same as filling it twice and you save 2 trips to the gas station. I never understand people that only put $xx in a t a time. Unless you have financial problems, fill it and be done.
I throw $75 at a time because most gas stations in my area cap at that per transaction and I don't want to waste my time for another five or ten dollars. But I like try to keep the tank full every fill versus multiple small full ups. I don't think I even put 100miles a week on the 4Runner.