Bolt ran 4.64 for 40m in Berlin and there are 36.55m in 40 yards.
Considering Bolt ran .86sec for that 30 to 40m segment, but was still accelerating, I'll subtract (.3 of .86)= .26 from his time of 4.64 to get 4.38 for 40 yards.
But that is an electronic time. In the 100m, the conversion rate for hand timing is .24, so that comes out to a 4.14 40 yard.
For electronically timed 40-yard dashes in football, the runner is allowed to start when he wishes (so no reaction time), and a timer hand-starts the clock, therefore the .24 conversion is legit.
Chris Johnson has the fastest ever recorded football 40 yd time of 4.24, but the fact that the finish is hand timed makes it somewhat unreliable.
I also think there is a difference when running a 100m to a 40yard. In the 100m, the acceleration is a little more controlled than in a 40yd.