Five years of timed 40s at my HS, which was a pretty big school-one kid who was a conference leader in the 100 ran a legit 4.5, another kid a couple of years later could run a 4.6, and he could fly-great athlete, could break off 50 yard runs. Those were the fastest.
Forty times have to be about the most lied about stat there is, even worse than bench presses.