Funny how often you read that in resignation posts. Four years is typically when one's shares become fully vested. At this point there's no way for him to make significantly more money from working at Stack Exchange.
It's perfectly logical to quit now, to either 1. Relax (maximizing family happiness) or 2. Start a new company (maximizing economic opportunity).
Considering how recently they pivoted into a VC-backed company it seems rather too soon for a co-founder to be jetting though. Wonder if Spolsky regrets backdating their vesting schedule (if that's what they did). Might be a bit of a cautionary tale for founders.