So if the company doesn't IPO in four years, and instead slowly staggers into the ground over ten while always promising it'll IPO in a few years, how does that change the terms?

Also, how liquid are these shares? If you can sell at IPO, cool. If you have to hold them for a year watching to see if the share price holds or (somewhat likely) collapses, not so good.

I'd take the salary and buy stock in public companies with it.