As many amateur carpenters soon discover, most lumber is slightly shorter than advertised. This is because the lumber may have been planed or finished on at least one or two sides, if not all four.
Lumberyards and home improvement stores use a certain code to let customers know how many sides of a particular piece of lumber have been surfaced. A piece of lumber marked S4S has been Surfaced on 4 Sides, in the same way that S1S wood has been surfaced on one side and S2S wood has been surfaced on two sides.
Because S4S wood has been planed or surfaced on all four sides, it will be noticeably less than the advertised dimensions. A two by four piece of lumber designated S4S may be closer to 1 7/8ths by 3 7/8ths or less.
The advantage is that all four sides are suitable for projects like decorative railings and fencing, since they have been planed down smoother than unfinished lumber. The disadvantage is that S4S lumber can be more expensive, since the lumber processor must make more passes through the planer.
Not all projects would be improved with the use of high-quality S4S wood, especially if the wood is destined to be hidden under a different material like drywall or stucco.
Woodworkers often prefer to use S4S wood for projects requiring carving, since the lumber does not need to be sanded smooth or planed beforehand. Framers may choose to use S1S or S2S wood where only one or two sides may be exposed to public view.