Gavin, if your daily round trip commute is only~4 miles, I doubt you will see a significant improvement in gas mileage. Bascily, by the time your truck warms up and reaches optimal efficiency you have arrived at work so you and of course the same for your retrun trip home. If you do this 5 days a week, hopefully you change your oil more often than the 5,000 mile suggested interval. Those short drives are not good for the longevitiy of an engine.