If you are withdrawing 4%, that seems to be pretty safe for never running out of money, but it might not cover your expenses year-to-year. Luckily, as a retiree, you have several options:
1. Withdraw more this year, then when economics pick up, work for a year and refill your coffers.
2. Withdraw 4% and work a small job part time to make up the difference, assuming you're a mustachian and not a boglehead. A boglehead needs the income from a $3MM portfolio, a mustachian needs about $26k-30k/year or a 650k-750k stache.
3. Since some months, you're able to withdraw more than you need, you're reinvesting anyway, so that should get added in. Of course if you retire the day before the crash, you're going to have to decide what to do with options #1-2.