ForceField Pro Sub 4 Back Protector F|1508-FF-1004-4M|0. This Forcefield Pro Sub 4K Ultimate Level 2 Strap-on Back Protector is designed to provide maximum protection and comfort for all seasons. It is made from high-quality PU Foam, PE Foam, Nitrile Rubber, and Neoprene materials, ensuring durability and flexibility.