as an employee, if you are lucky/skilled enough to end up at a successful startup, and you aren't very careful with tax issues, you can find yourself stuck: if you leave, you have to exercise, and immediately owe hundreds of thousands of dollars (or more!) on a completely illiquid asset that you can't sell. Which doesn't even take into account the potential for that asset to become less valuable.
That's not a decent way to treat people. Startups don't write the tax code, but many are willing to take advantage of it to control people this way.
After you put your 4 years in, you should be free to re-up or leave. Not free to leave if you are willing to risk all your liquid assets and/or borrow heavily.