Perhaps (I am speculating, not dictating) this is exactly why four-year vesting periods work. If a large chunk of people burn out or get bored after four years,

Perhaps (I am speculating, not dictating) this is exactly why four-year vesting periods work. If a large chunk of people burn out or get bored after four years, a five year vesting period would create sullen, unproductive boat anchors. A three year period would have people jumping ship to found something new while they are still able to bring their prowess to bear growing the company. But after four years, both parties have maximized their gain from each other.