When it comes to wood or lumbering, S4S means 'Surfaced on 4 Sides. What this means is that the material on all 4 Sides' is finished. Dimensional lumber is usually S4S. S4S finished lumber and dimensional lumber are labeled based on the size of the rough board before it was planned smooth or the nominal size. For instance, 1 x 6 is ¾ x 5 ½.