When he hits top speed is irrelevant. Yes, he would still be accelerating beyond 40y mark, as would 99.9% of world class 100m guys. The sub 4.0 was referring to a time based on the methods used by most people and NFL scouts, hand-time. In that method there is no reaction time accounted for, the clock starts at the starters notice of first movement and is stopped when the starter "sees" them cross the finish line. LOTS of room for error. It would be a bogus time for sure as are any other 40 times timed in this method. The proper method is FAT and that is still based on first movement, where as in track the clock starts when the GUN GOES OFF. FAT is what a lot of the camps use and why you don't see any of the 4.3's like everyone says they run. Bumphis ran 4.5 or so at one of these. So would Bolt run a REAL sub 4.0 40? No. Neither would you see these NFL and college 4.3's
FYI - Per his coach, Ben Johnson was timed by NFL scout in the 80's at sub 4.0 HAND-TIME, like all the NFL scouts do, so was one of his training partners. Usain Bolt would be with Ben Johnson at his prime step for step.