Plus fours are breeches or trousers that extend 4 inches (10 cm) below the knee (and thus four inches longer than traditional knickerbockers, hence the name).
Plus fours were introduced in the 1920s and became popular among sportsmen--particularly golfers and game shooters--as they allowed more freedom of movement than knickerbockers.
An “extravagant, careless style that fit right in with the looser fashions and lifestyles of the 1920s,” plus fours were introduced to America by Edward, Prince of Wales (later Edward VIII) during a trip in 1924.
Here below is a set of vintage photos that shows men in plus fours in the 1920s and 1930s.