To determine whether your S-reg Renault Megane has a K4M or K4J engine, you can check the engine code, which is typically found on a label under the hood or on the engine itself. The K4M engine is a 1.6-liter 16-valve unit, while the K4J is a 1.4-liter 16-valve unit. You can also refer to your vehicle's manual or service history for specific engine details. Additionally, checking the VIN or contacting a Renault dealer can provide clarity on the engine type.