Definitions for "S4S"
An Industry acronym for “Smooth four sides,” which is commonly used when referring to wood characteristics. If a piece of wood is referred to as S4S, it will have four, finely sanded sides
A board that has been planed so that it is smooth on all four sides.
'sanded four sides' or just plain smooth wood. When a board starts off at the lumber yard unsanded or 'rough', it has a true size of say 2" x 6". Once the happy sanders do their job, you end up with a 1-1/2" x 5-1/2" board- go figure