Bo Jackson was a freakish athlete and a helluva player whose career ended far too soon. However, some of the urban legend around that guy has gotten out of hand.
I am not doubting the OP. I have no doubt that somebody at the draft combine said that he ran a 4.12/40. I just think that person was mistaken or not telling the whole story.
As someone who actually saw him play, he was a great player but, aside from Techmo Bowl, he wasn't the best RB I've ever seen. Also, he's not the fastest player I've ever seen or even among the top five fastest players I have ever seen play.
Again, that is not to say that he wasn't incredibly fast - especially for a bigger guy. As I said, he was an absolute freak of an athlete and those don't come around all the time.
I am just saying that he was not as fast as Deion Sanders, Darrell Green, Chris Johnson, etc. Now, he was a solid 30-40 pounds heavier than all those guys so take that into account too. However, from a pure speed perspective, he was a notch down.