I believe many, many people have probably run a sub-4 in history. Here are some of my thoughts why.
First, it's easy to have the attitude that we are a special generation, and becaus it's rare for us it must not have been done before. Consider just the shear number of humans who came before us. Consider how inactive we have become in the last few centuries as a population. Consider how a lean body used to be the norm and fatness a sign of wealth, unlike today's normal. Consider the lack of distractions in the past- no tv, Internet, newspaper, etc. consider how this would have lead people to play more physical games for fun and entertainment. Especially think of these scenarios pre agriculture when people were not working so much and play happened more.
In all that time, with all those active people over countless generations, at a time when speed and endurance were crucial to survival of the gene pool, think of how many quick runners there would have been. All you need is one in a million, or one in a hundred million of those people to be a genetic freak and the right circumstances and there would be thousands who have broken 4 in the past. It could be running from a predator, a race, for the sheer joy fun. There had to be some nut jobs back then who just ran and suffered out of the shear enjoyment.
Fossilized human footprints are rare, but I recall reading that a few were discovered in the past decade, and the speed they calculated from them was something like 4-5 kph below Bolts average 100m speed. Now if that is just the tiniest sliver of human running history they concluded it was very likely there were humans capable of beating bolt thousands of years ago.
And lest us not forget if we open it up to our ancestors beyond homo sapien sapien, we know of at least one group, either Neanderthals or cro-magnons (I can't remember) who were not only smarter, but stronger, larger and better athletes as well. Funny how they died off and not "us".
I'd love to know the truth, the real first one to do it. Race people of the past. Wouldn't that be neat?