I think any talented male runner given one year of hard core training could get down under 4:40. I say that only because I went from an "ugly duckling" 5:35 mile during freshman cross to a 4:37 by the end of track season. My only previous experience was the 200, 400, and long jump in middle school. Of course then I ran my HS PR the following year in 4:27 and improved only a few tenths by my senior season.
My point? He could be ungodly talented and run 4:00 by his senior year...or he could have trained his ass off for that 4:35 and might get down to 4:20 by senior year.
Alan