He was the fastest guy on the team last year. Only unseated of that title because of the arrival of a certain freshman with world class speed. He runs in the mid 4.3 range. He has blazing NFL speed. A RB faster than 4.5 is blazing. Most of the starters at RB in the NFL run north of that number. Jamal Charles was the exception. Fournette is a freak of nature.
When you're talking in tenths of seconds, you're really just splitting hairs unless you're running track. Anything under what, like 4.5. is blazing.
Chris Johnson ran the fastest 40 ever at the NFL combine. His time was 4.24. Bo Jackson ran a 4.12 40 at the Superdome at the 1986 combine. That was before "more modern electronic timing." When Usain Bolt set the world record in the 100 meter dash he ran the first 40 yards in 3.94. Still, for somebody with Fournette's size anything faster than 4.4 is simply amazing.
I don't give a rat's ass about the 4.4 v 4.2 as it is not run in pads. The eye test says LF is blazing fast and he outruns other fast or allegedly faster guys - in pads.