A 4″ × 3″ flexible rubber coupling is a plumbing fitting designed to join a 4‑inch pipe to a 3‑inch pipe, often for transitioning between sizes or materials such as cast iron to PVC. It is made with an elastomeric (rubber or neoprene) sleeve and stainless steel clamps for a secure, flexible, and corrosion‑resistant connection. Proper installation involves ensuring compatibility of pipe materials, correct clamp torque, and proper alignment to maintain a leak‑proof seal.