Does compensation continue at the year 4 level until your next raise?
Do you take a massive pay cut in year 5?
Has anyone ever stayed at a tech company for more than 4 years to find out?
In year 5, your initial grant would have expired, but your new grants would be vesting, so you would continue with 4 GSU grants vesting simultaneously, and you would stay at the same income (or better, or worse, depending on the size of your grants, the value of the stock, etc).
The idea is that your salary would be ~50% or less of your total compensation, once you factored in the GSU stock grants and the yearly bonus. It was so complex there was an internal HR site (go/totalcomp ?) that would try to break down your compensation.
The process is designed so that you reach "max" income in year 4 which does not substantially decrease, thus locking you in. Because if you left, it would take 4 years of tenure at the new company before you built up 4 simultaneously vesting grants.
All this is assuming you meet or exceed performance targets.
Frankly, I far prefer Netflix, where they just pay you every 2 weeks, and you're not waiting for grants to vest, or for yearly bonuses.
Note the so called "cliff" after year 4, it's your cue to look for a new job. Especially if your initial grant was relatively large compared to refreshers as is often the case, but stock did not perform too well to compensate for it.
Size of refresher grants depends on your level, review and stock performance - company usually has some target $ amounts in mind for them.