Stember was a great runner, but according to this profile, that 4:04 came in 1995 and he finished high school in 1996:
From what I could find, the top high school miler in 1994 was Meb, who won the National Scholastic Invitational mile that year in 4:05, with Seneca Lassiter second in 4:07.
I think it’s difficult to pinpoint one runner or year as the beginning of the renaissance of high school miling or, more broadly, US distance running. Many will understandably say it started with the Big 3 in the 2000-2001 school year, but there were notable runners who starred in the decade before them such as those mentioned in this thread plus Andy Powell (4:02 mile as a high schooler in 1999), Jorge Torres, Adam Goucher, and others.
At the pro level, we had Bob Kennedy, Todd Williams, and Marc Davis running well in the mid to late 1990s. Ritz has stated that Kennedy inspired him. So the Big 3 were inspired by their predecessors, who were inspired by those who came before them. It’s a lineage.
The most significant development in the renaissance of American distance running was probably the internet. Here’s what Kennedy said about it, when commenting on how high school distance running changed in the 25 years since he had competed in high school:
“Twenty-five years ago there was a big void in information, meaning we'd be waiting for Track & Field News to come out every month before you knew what the guys in the rest of the country were doing. Now it's instantaneous with the Internet of course. So, what I think that has done is raise the level and depth of competition at the high school level just because you know what you have to do. Now I know I need to run 9:10, 9:05, 8:55, whatever, and that changes how you approach your training -- and your racing, ultimately.”