4% is the standard number that's quoted by pretty much any financial planner. It's based on backtesting and assume that there's be large downswings along the way.
At $4mm in a market account (not 401k), you also have the option to take out margin loans at shockingly low interest. This gives you untaxed cashflow without touching your principal. 160k untaxed is a lot more cashflow than the same number in salary or retirement distributions.