This is probably a troll, but I'll bite:
I ran 4:48 for 1600 in 8th grade. I ended high school with a 4:25 mile PR and a 9:35 3200. My current PRs are 4:15 mile, 14:29 5k, 1:09:45 HM, and 2:30:10 marathon. Those are my PRs, but the guy who beat me in 8th grade with a 4:46 washed up in high school.
I've come to the conclusion that it is incredibly difficult to predict lifetime PRs from a middle school race for two reason: puberty and dedication to the sport.
Some middle schoolers are literally doped up athletes. Puberty can turn a 6 minute miler into a 4:40 guy overnight. With this in mind, if the 4:48 came as a 7th grader who is already shaving every day, then your potential is probably about 4:30 in the mile. If the 4:48 came as a scrawny 4' 10" kid, then the limit is probably sub 4:10 easy.
Additionally, middle school phenoms are often unfortunately pushed by parents or coaches who are not motivated by love of running. Running is a sport that rewards consistency. If you are not motivated and you do not put the work in, then it is doubtful you get through high school without mentally burning out. If you truly love the sport and you truly love putting in the miles and grinding through the workouts and trolling on letsrun (loving the running lifestyle is a huge part of the sport), then the sky is the limit for your PRs. More importantly, if you love the sport, then you should not be concerned about your physical potential; you will achieve a lifetime of rewarding memories from the journey no matter what your PRs end up being.
Good luck!