Fill up at a gas station. Record your odometer reading. Drive fifteen or twenty miles in thrid, then turn around and go back and fill up again. Record your gas usage and your mileage. Do the same run again, this time in fourth gear. Fill up again and compare the difference....this way you'll know for sure.
IMO it's better for the engine to be running on cam at 2000 than be labouring at 1200 for any length of time.
If you go through this exercise , please post the result, I'm curious too.....