Between charging at home and charging at work, it shouldn't be necessary to occupy the charger every single time you pull in. Maybe once or twice a week, depending on your commute. A long range Model 3 has as much range as your average gasoline vehicle... how often do most people fill that up?
Seems to me if 4000 employees go electric and you can slow charge anywhere from 5-30 miles' worth of power per hour at a worst case scenario and up to 44 at the best, then it stands to reason not everyone has to plug in every single day. In fact, I'd argue most folks would only need to charge up once a week, and for those with very short commutes, once every 2-3 weeks.
I think 250-500 chargers for 4000 vehicles would likely be sufficient to keep everyone charged up and moving around.