Interesting. a 3 second discrepancy between 400m and 1500m PR's. Imagine if world class athletes could pull that off. We'd see El Guerrouj running sub 3:10 for 1500m!
I think some of the posters here have undeveloped sprint speed, or perhaps set their 400m pr's while training for a much longer distance event.
With that said, I do agree that 60 second quarter speed is plenty to break 5:00. Me personally...61.5 and 4:49.
I ran a 4:59 off of a 67 400m PR
As a poster above said - nothing to be proud of. Good endurance or poorly developed speed. Ultimately most elite athletes have some speed, and work to develop it to some degree as well. The alternative is a huge amount of endurance work - which is easier said than done. Even a 1500/5k guy like Ingebrigtson has run 51 and 1.46 for 400/800m, which is not particularly fast for that level, but he also he has to run over 100 miles per week to do that. Some with better speed could get away with less volume (although only to some degree) if they trained optimally.