Based on his 10m splits, Ben Johnson came through 40 yards in his 9.79 about 4.22 seconds after he left the blocks (so not counting his reaction time of 0.132 seconds).

Based on his 10m splits, Usain Bolt came through 40 yards in his 9.69 about 4.19 seconds after he left the blocks (so not counting his reaction time of 0.165 seconds).

Based on his 10m splits, Usain Bolt came through 40 yards in his 9.58 about 4.20 seconds after he left the blocks (so not counting his reaction time of 0.146 seconds).

(Bolt coasted his last 10m during the 9.69. It was faster from the blocks to 40m than the 9.58.)

And yes, I estimated 40-yard times (using the times to 30m and the times to 40m), rather than using 40m times, for those of you wondering if there is an English/metric thing going on.

They broke down like this for reaction time/first 40/last 69.36/total:

0.132/4.220/5.438/9.79

0.165/4.187/5.338/9.69

0.146/4.200/5.234/9.58

The reaction times are available to the nearest .001 but the times to marks are only available to the nearest .01.

In case you're wondering how much knowing they're going 100m slows them down, Ben Johnson's best indoor 50m was 5.55 and he went through 50m in his 9.79 in 5.50. His best indoor 60m was 6.41 and he went through 60m in his 9.79 in 6.33. The current world record for 50m is 5.56 at altitude and 5.58 at sea level. The current world record for 60m is 6.39.