Here are all the sub-4 hs'ers in the U.S. listed on wiki, together with their pro pr's at the same distances, plus some additions for xc for recent graduates. It shows that the vast majority improved at least in college. Not too many, like Slagowski and Danielson, went backwards or stopped running.
Alan Webb 3:53.43/3:30.54/3:46.91 AR May 27, 2001 Prefontaine Classic
Jim Ryun 3:55.3/3:33.1/3:51.1 WR June 27, 1965
Colin Sahlman 3:56.24/3:32.97 May 28, 2022 Prefontaine Classic
Owen Powell 3:56.66i February 22, 2025 (xc redshirt)
Josiah Tostenson 3:57.47i/24:31xc February 22, 2025
Simeon Birnbaum 3:57.53/3:52/3:37.02 June 1, 2023
Hobbs Kessler 3:57.66i (3:34)/3:29/3:46 February 7, 2021
Drew Griffith 3:57.72/4:00.58 (22:53xc) May 30, 2024
Drew Hunter 3:57.81i/3:54.8/3:33.41 February 20, 2016
Gary Martin 3:57.89/3:48i/22:19 xc June 2, 2022
Rocky Hansen 3:58.24/3:38.7/3:57.90/13:07/22:18 xc June 1, 2023 ACC xc champ
Quentin Nauman 3:58.65 June 5, 2025 sr xc state course record
Tinoda Matsatsa 3:58.7/1:45/3:54 June 1, 2023
Connor Burns 3:58.83/4:01/23:44xc June 2, 2022
TJ Hansen 3:59.02 May 24, 2025 redshirt (15:18 5k xc)
Jackson Heidesch 3:59.08/- June 1, 2023 Duked
Corbin Coombs 3:59.23 May 24, 2025 on a mission
Reed Brown 3:59.30/3:36/3:54 June 1, 2017
Rheinhardt Harrison 3:59.33/3:38.70/3:54.74 June 3, 2022 despite health issues
Matthew Maton 3:59.38/3:39.97/3:58.34 May 8, 2015 Became a lumberjack
Grant Fisher 3:59.38/3:33.72/3:48.29 June 4, 2015
Tim Danielson 3:59.4/- June 11, 1966 San Diego Invitational
Simeon Birnbaum 3:59.51/3:37.93 hs/3:52.81/3:37.02 pr’s June 15, 2022
Michael Slagowski 3:59.53/4:10 April 29, 2016
Leo Daschbach 3:59.54/3:55.64 May 23, 2020
Tayvon Kitchen 3:59.61i/23:11 xc February 22, 2025
Zachary Hillhouse 3:59.62/- June 16, 2024 Can’t find any college times
Lukas Verzbicas 3:59.71/tri after 62nd in a xc race June 11, 2011 Adidas Grand Prix
Tommy Latham 3:59.79/23:57 xc March 28, 2025
Marty Liquori 3:59.8/3:52.2 June 23, 1967 AAU Track
JoJo Jourdon 3:59.87i/3:37/3:56.37 February 3, 2024 at Wake