Running under a 5 isn't world class or something extravagant. Under a 5 on track really isn't out of the ordinary if you are in shape and train for it. So much of the start and get off is form. You get someone to correct your form and you'll shave .1-.3 seconds off your time. I'm sure some of the young bucks around here can break 5.0 flat.
4.5 on the other hand is another story.