Bo Jackson was 4.12
Bo Jackson ran a 4.12 but it was not electronically timed and therfore could not be verified. Chris Johnson ran a 4.24 at the NFL Combine: The fastest electronically timed 40 yard dash.
According to several online sources, Bob Hayes' time in the 40 yard dash wasn't recorded. See The Fastest 40 Yard Dash Ever.
4.3
Chris Johnson
The NFL's fastest official 40 yard dash time was 4.25 seconds by cornerback Fabian Washington.
At the combine prior to his rookie season, he ran a 4.64 second 40 yard dash.
my brother goat sets the record for best 40 yard dash time and its 4'01
the fastest man alive can run it in 3.46 seconds from a running start
De Anthony Thomas can run a 40 yard dash in 4.21 seconds
The fastest 10-yard split ever electronically recorded was the 1.40 time recorded by current Tennessee Titan runningback Chris Johnson at the 2008 NFL Scouting Combine.
Steve Smith