Lumber is initially cut to the listed dimensions. a 4"x8" timber is cut to 4"x8"... this is the rough cut size. This is often too rough (and in the case of new lumber, also too wet) for most applications.
The wood can then be surfaced... sanded or planed to produce a smoother surface, which is more appropriate for most interior uses. This removes about 1/8" of material, which means your end result is about 1/4" smaller each way. A piece of wood that started out cut to 4" x 8" will still be called a 4x8, but it will actually be slightly smaller... closer to 3.75"x7.75", because it is actually a surfaced 4"x8".
At WRT, we can cut to whatever size you require. If you want a piece of wood that is a full 4" thick after surfacing, we can roughcut a piece that is 4.25" thick so that it will be 4" after surfacing.