I suggest that you read up on the Trinity study, but the short answer is this:
In most historical cases you can withdraw money even during very bad down markets and you will be just fine. The reason has to do what the markets do on average.
Adjusted for inflation and including dividends, the SP500 has returned about 7-8% annually over almost all 20+ year periods. If you are using a 4% WR on average your portfolio will keep growing. However, there have been years when the market drops 30%, and years when it gains almost 40%. Most of the time big drops which reduce your overall principle are then overshadowed by big gains which increase your portfolio. The biggest worry is that a large market drop will occur very early into your retirement, when you haven't gotten the advantage of it increasing slightly. Even then, though, most of the time it will still recover.
A key way to mitigate the uncertainty is to be flexible. When you retire, if the market takes a huge drop, consider cutting expenses back a bit, or earning some money through a part-time job. For someone living off of $40k in retirement, as little as an extra $5k/year in reduced spending or extra income could allow all portfolios to survive the worst historical market conditions and come out smelling like roses on the other side. Another methods is to save up even more money, and use a WR that's even smaller. A portfolio with a 3% WR has never failed during any time period. The downside is that reaching such a large portfolio may require working several years longer than you otherwise would.
Hope that helps. Remember that when we talk about historical scenarios, we're talking about all periods of the SP500's performance over the last 100+ years. It's always possible that the next 20, 30, 40 years might yield market conditions that are worse than any previous 20, 30 or 40 year period. again, that's where flexibility comes in.