The way it actually works is that when you join, you get a starting grant that vests over next 4 years, and then every year, you get a so-called refreshers, each of which is roughly a quarter of your original grant, which also vest over 4 years. This means that in year 4 of your employment, you get a (last) quarter of your original grant, along with a quarter of each of the 3 refreshers you received in the next 3 years. Then, in year 5, your original grant is gone, and you only get a quarter of each of the 4 refreshers you got in past 4 years.
Depending on the size of your original grant, and the size of refreshers, this can result in some pay cut in year 5. This pay cut can be bigger or smaller, depending on the size of your original grant, size of refreshers (which are driven by your performance rating/promotions), stock price in year 5 etc. However, other than some special circumstances where you get some additional discretionary stock grant, after year 5 you kinda reach a steady state, and your the value of your stock compensation can go down pretty much only if the stock price goes down (assuming constant performance over time).