The 40 is a funny thing... it has value as a metric but is often overemphasized.
This article (United States Olympic Committee - The NFL treats 40-yard dash times as sacred) is pretty fascinating and tears down a lot of the popular ideas about the 40.
I always get a good laugh out of that article. It clearly states that the race was broken down into ten meter intervals, lists the sixty and fifty meter times--then lists the 40 time as yards. It was a metric breakdown.
Johnson actually ran 40 meters in 4.38 seconds. Anyone care to calculate just how many yards that was?