Rules of thumb are an easy way to quickly see where you stand financially. Once you reach 25X your spending in liquid net worth (the 4% rule presented as a multiple of spending) you are assumed to have enough to retire under the 4% rule, regardless your age.
However, as we are seeing with the current market turmoil, the simple rule of thumb has one fatal flaw. If you reached your 25X goal a few months ago and decided this was the time to step away from traditional labor, you now face a withdrawal rate from your index funds a third higher than expected. This will reduce the account value early in the distribution phase, lowering the total amount you can get from the investment over your lifetime.
Another rule of thumb is to keep 6 months of spending in cash in case you become unemployed. Under a normal job loss or economic decline this would be a reasonable policy to follow. Unemployment insurance can provide additional cushion to the 6-month cash reserve.
Black Swan events (unexpected negative economic events such as the housing crisis or pandemic) throw the whole rule of thumb out the window. Black Swan events do not happen often, but they do occur every decade or so. Looking back at U.S. history, it seems something always happens every decade to knock the markets lower and slow economic activity. The 2010s are the only decade to avoid that fate and 2020 seems to be making up for the oversight.