i could completely buy 6 to 5. that's your raw kid shows up who didn't play sports before, suddenly is running and getting in shape, knock the rust off and get them some legs and lungs, he's a solid 5 minute kid. requires no special talent.
the problem with 5 to 4 is if you are god's gift most of the time you would show up faster than 5, even as a noob. and even then 4 is a tough ask.
re ryun, stuff i read says he ran as a fish and soph. that makes his junior year his 3rd season, not "within a year." sounds to me like 3 years of XC and TF. which is less than most, and a pretty incredible trajectory to sub 4, but not some complete noob doing it in a year.