There is also a question of how efficient a mile is. When talking about transportation efficiency, the distance you want to talk about is the distance from poin

There is also a question of how efficient a mile is. When talking about transportation efficiency, the distance you want to talk about is the distance from point A to point B. When you talk about miles per gallon, you generally are talking about the total distance traveled which, unless you are moving in a perfectly straight line, is greater than the distance between the two points. I assume that air travel would travel closer to a straight line, so a single air mile gets you further than a single bus mile. I haven't actually checked this, and have no idea if this difference is large enough to cover factor of ~4 difference you are citing.