Most of the milers listed in this thread have been 'great' or 'excellent' milers. Not 'good' milers. There's probably not a single 'great' miler that couldn't break 50, but I'd be willing to bet there are hundreds of 'good' milers that don't have anything better than 52-54 in their legs.
Assuming these rough standards:
Legendary: sub 3:48
Great: sub 3:51
Excellent: sub 3:55
Good: sub 4:00
Having been a 'good' miler myself and barely clocking a 51 in my prime (speed-wise) and doubtful of my ability to go under or near 55 when I was out of my 'speed' prime but in my 'endurance' prime (but still a good miler), I think I can speak from experience.
I bet you'd find absolutely incredible 400m range in guys who are 3:55 - 4:00 types. And I bet most would fall in the 51-53 range.