Easy, I give you X million dollars and take over your company if you stay at Google 4 years.

Sign here:

An acquirer can't literally force someone to stay on as an employee for 4 years. What they typically do is set up a deferred compensation scheme (golden handcuffs) with a large guaranteed bonus payment or stock vesting at the end of that period. In some cases they also include a non-compete clause in the agreement which leaves the employee with few other employment options, although those are only enforceable in certain jurisdictions.

It was probably in the terms of the acquisition itself