S4S is a short way of saying that a piece of wood has been surfaced on four sides. These four sides, are the two large faces and the two long edges. The ends will still be rough.
Pieces of wood that are S4S are going to cost more than rough sawn or faced lumber because more was done to the wood before it was brought to market.
The more work a board has to go through, the higher the price when its sold. That’s just how the world works, but if you don’t have the tools just pay the extra money until you can afford them.
For more about woodworking, please take a look at 13 Myths About Getting Into Woodworking