Speed is the limiting factor in any race, obviously the longer the race the less of a factor it is. However, even the world's best marathoner had good credentials at shorter distances and some of them where even world class miler in the early part of their careers. Notwithstanding the recent athletes who went straight to the marathon, Bill Rodgers, KK and Hall are the only world class marathoners I can think of who did not have success at shorter distances. The point is, how fast you can run at a shorter distance gives a good indication of your potential at the next distance. Drew is definitely strong, but perhaps no more so than Lindgren, Pre, Virgin, Ritz, Rupp, Solinski, Fernandez, LV, Ches or Fisher, but unless you can run a sub-1:50 or thereabouts, there is almost not way you are going to run a 3:55 mile. Ryun and Webb were able to run super fast miles in H.S. primarily because they were also great 800 runners. I am sure Drew can run a good 800, but I am almost certain he is not a sub-1:50 type for the simple fact that there is almost know way a H.S. kid who thinks he can run a fast 800 is not going to run one...athletes always think they can run faster at a shortern distance than they can and don't pass up the opportunity to prove it.
With all of that said, Hunter has far more upside in the 2-mile, I can see him in the mid-8:30s and about 4:02 in an indoor mile and just under 4 outdoors. Those are reasonable expectations based on what I have seen so far.