The person who told you "I'm looking because I just hit my first-year cliff" actually told you "DO NOT HIRE ME". Listen harder.
People do leave when they hit four years. Four years is a long time! Some teams are O.K. with this, but if you're not, there's no reason to mess with vesting to solve the problem; just grant them more of the employee pool to stay.
Everyone is always looking for better opportunities. That's fine. Be the best opportunity for everyone on your team, or get better at recruiting. Vesting can't help you with this problem, but it sure can hurt you.