You are over complicating things again. Hypothetically, if someone is truly at max velocity after 4 seconds of running then they aren’t accelerating properly and that should be their training focus, not what you mentioned. Again, I think you aren’t getting what acceleration actually is, forget what the people actually look like approaching max velocity and just thing about splits. I can’t imagine anyone truly being at their max velocity after 4 seconds of running.
Anecdotally, when people hit max velocity later in the race than expected its because they have had a terrible start and spent 10-20 metres not actually doing a lot.