If your son attended a PG tournament, or if PG has seen your son at some event, chances are, he will receive some kind of ranking and will have a profile on the PG website. The more PG sees them, the more accurate those rankings will be. In order to get a grade and usually, a picture on the profile page, you must attend an individual showcase. Ranking and grades are completely different.
And from what I remember, there are likely more than just 4,000 ranked. You will notice that once you get past the top 100, there will be many players that are "tied". In other words, there may be 100 kids listed as ranked 500. There may be 500 kids ranked 1000. Hope this helps...