It is usually the salary + stock award that will vest over 4 years. For most companies with a flat vest, the total will come to 500k/yr for certain levels.
The appreciation is all yours. Future perf bonuses which will be each 6 months or a year are on top of this. Some companies do adjust for what you're making in that year and cut down the perf bonus (In a previous company I once got a promotion without any stock because our stock had tripled in the time...) - but these companies have a serious growth outlook. Once the stock flattens, there will be a major drain and they know this.
On average over a long period, but in this case you are invested in one company over a very specific entry date and period of time, with some critical periods blocked from selling.
Facebook peeked around the end of July 2018 at 209$, if you got stocks then you might be in trouble, or already sold out after seeing 124$ during December 2018.
I think the FAANG people I know who supposedly make 500k/yr only got to those numbers by never selling a single share and then reporting total net worth including appreciation divided by time worked. Their base salary isn't actually that crazy. People who sell all their shares for index funds didn't do as well.