The S4 is the russian's version of the C4. It is very effective. It does 2x the amount of damage (3x if you have 35+ kills. ) It also temporarily blinds and slows all enemies within 25 ft. From it. To get the S4, you first have to have a class with 2 flash or concussion grenades and one C4. Then, throw both flash/con grenades and make sure they both affect someone. Without dying, you can now throw your C4 which is now an S4. You will unlock the achievement "Explosions in Russia" and get the trophy "Power Blast".