For decades, fitting guides taught: "Measure under your bust and add 4 inches to get your band size." This came from a time when bra fabric had zero stretch.
Why it fails today
Modern bras use 5–10% elastane in the band. They are designed to fit your true under-bust measurement and stretch slightly during wear.
The result of using +4
A band that is 4 inches too big provides almost no support. The straps end up holding everything, leading to shoulder grooves, neck pain, and the bra band riding up the back.