Former 47.8 400m runner.
Hardest indoor workout we'd do, during our toughest phase in Jan.:
4/5x(3x200m) 1st 200m @ extensive tempo, 2nd @ intensive tempo, 3rd all out. 30-45seconds rest between reps, 5 minutes between sets. I would typically do it in 32 seconds, 29/30 seconds, and 25 seconds.
Hardest outdoor workout we'd do, typically done in April during the last big buildup phase of the season:
6x250m going through 200m @ 99% effort. I would usually go through 200m at 22flat when I had a PR of 21.7. Rest was until we felt ready to go again.
We were one of the top 3 DIII 400m schools a while ago. We had a top 4x4 team one year. My last year we had guys with PRs of 47.4, 47.9, 48.2, 48.6, and 2 guys in the mid-49s.