If you think that humans *can* condition themselves to that level. Speed 4 is the speed and reflex time of the Great Cats. If you look in the Bestiary, there aren't any other natural creatures with Speed 4. To be Speed 4 your reaction time is on a par with these animals. You're Tarzan, Doc Savage, Conan the Barbarian. Literary heroes.
Not Usain Bolt.
I mean, perhaps the world's greatest ever sprinter, but if he were to try and out-manoeuvre a hungry lion..?