Also called: 4% rule, SWR
The share of a portfolio you can withdraw in the first year, then raise with inflation, without running out over a set period. The 4% rule comes from 30-year US historical periods.
Bengen's 1994 study found that a 4% initial withdrawal from a balanced portfolio survived every 30-year period in the US record. It is a historical finding, not a guarantee, and it assumes fixed real spending regardless of markets.
RetIQ's % of Portfolio mode is different: it withdraws a fixed percentage of the current balance each year, so spending moves with the market.