A. O. Smith S4 is a 100% RO water purifier with a 2-year warranty on the RO membrane. It incorporates a 5 stage purification system, along with a Side Stream RO Membrane Technology. It uses one of the latest technologies that is utilized in the water purification process.  A. O. Smith S4 RO Water Purifier works under the principle of RO (Reverse Osmosis) technology which is one of the most effective process for treating water.