That is one of two ways to look at it. Most employers do not think of the initial stock as a grant that you have earned for signing on and then four years of dragging out payment. They look at the stock as four years of payment that, along with your salary, is designed to keep you motivated the entire time. So, the other is:
zynga: want to be an early employee in our risky little venture?
emp: um - the salary is kind of low and there's no job security
zynga: we'll toss in 25,000 options per year for the first four riskiest years. potentially worth $2.5M per year if our stock hits $100 in the ipo
emp: okay, that sounds better. i'm in
2 years later
zynga: hey, looks like you did pretty good the last two years, but the trajectory of the company has changed, there is no longer any risk in it for you, and it doesn't make fiscal sense for us to pay you $2.5M a year in stock anymore. We'd like you to stay on, though, and you'll still get a competitive salary and benefits plan. If you don't think that it is competitive, we understand.
emp: works for me. (OR) doesn't work for me.