I think there are probably thousands of kids who have the talent to break 4. For the vast majority of them though, if they start running in high school, it's already too late. There's a whole lot of people who could be great runners but they don't enjoy running, or do another sport, or even just another event. Brazier probably could've broken 4 in HS if he trained for it. If Fisher didn't quit soccer, he wouldn't have broken 4 in HS.
So yeah, if every kid in your high school was getting high quality training since elementary school, I think a dozen or so seniors would have a good shot at breaking 4.