My PR for 1500 is 3:42.1 (post college) which is near a sub 4 mile.
I was a sprinter in high school, ran 50.2 senior year.
Trained for the 800 for 4 years then moved to 1500.
Averaged less than 50 mpw and ran 3:42.9.
Averaged about 60 mpw the next 3 years and ran 3:42.1.
Peak workouts:
8X400 90sR 59s
12X400 75sR 64s, #4,#8,#12, 60, 60, 56
800, 400, 300 90sR- 1:59, 58, 42
4X400 5minR 55, 55, 55, 53
5X300 90sR, 41-38
12X200 60sR, 28s, finish in 24/25
Base Workouts:
5X1000 3minR, 2:57-2:39
16X400 75sR, 68s
3-4X mile, 4:50-4:35
Long hills, short hills, Fartleks
10-12 mile for the weekly long run
Drills, hurdle walk-overs thru the year
Anyway, there's one example