My own opinion, is that if you push the loud pedal hard on the 4 banger, it will end up getting about the same gas mileage as the V8. It just takes a certain amount of energy to accelerate the car to the speed you want. And, as someone else mentioned, the V8 is working less hard, so can be expected, if all else is equal, to last longer than the 4.
Mustangs are all about rumbling V8 power, but that's just my opinion.