只对问题不对人

This question really doesn't have much meaning... but I'll provide an answer anyway.

If you vested all your shares 4 years ago you are likely Mark Zuckerberg. FB was started in Feb 2004. It takes 4 years to fully vest on a normal schedule... so Feb. 2008... and four years after that is Feb. 2012. Mark gets stock, not options... so it doesn't really apply to him. So as far as I can see, this question wouldn't apply to any of the actual employees (yet).

That said, if someone has fully vested then there are two paths. If they are still employed by FB then they will be restricted in when they can sell to particular windows after the IPO. This period is negotiated with the investment bank and is usually not shorter than 3 mos. In addition, it depends on who you are... if you were an early investor then it's possible you can sell before the "normal" employees. If you got friends&family options, you can flip it the same day. Also, there will also be lockout periods around corporate reporting periods. If they left FB (which many have after 4 years) then they can sell at any time after the IPO. In fact, they don't even have to wait for an IPO... there are private markets for their shares springing up all the time. The legality of these private markets is still to be determined ... but since GS is behind them, they will likely prevail.