Strategy 1: Stay for four years and vest your entire initial grant. Strategy 2: Stay 1-2 years and vest 25%-50% of your initial grant, and repeat 3x-1x.
If all the companies you work for have similar likelihood of doing exceptionally well, you've paid 4 years and gotten either one lottery ticket worth X (strategy 1) or 2-4 lottery tickets each worth 50%-25% of X. When you don't already have "enough" money, 25% of X is still life-changing, and if you can get 2-4 shots at it instead of 1, that can be a very workable strategy.
If all the companies you work for have similar likelihood of doing exceptionally well, you've paid 4 years and gotten either one lottery ticket worth X (strategy 1) or 2-4 lottery tickets each worth 50%-25% of X. When you don't already have "enough" money, 25% of X is still life-changing, and if you can get 2-4 shots at it instead of 1, that can be a very workable strategy.