There's a huge difference between staying 15 years at a place, because it's a nice place to work and fits your life situation, and then signing a contract to stay some place for 4 years, when you'd much rather be doing something else.
Depends how much your golden handcuffs and presumably Google options on top are for those 4 years.
That really depends on how much you hate working there.
It is when you've got the startup bug, think of what you could built in 4 years. In OP's case he built picnik
4 years is a long time imho. 15 years with no stocks or multiple promotions is a time wasted.
Unless you, uhm, enjoy the work?
4 years is quite a while to work some place in tech nowadays.