The FJ400 is a model of a four-stroke engine commonly used in various applications, including motorcycles and recreational vehicles. It is known for its reliability and efficiency, making it a popular choice among enthusiasts and manufacturers alike. The engine typically features a single-cylinder design, which contributes to its lightweight and compact nature. This engine is part of a broader category of engines that includes other models like the FJ600 and FJ1200. The FJ400 is often praised for its smooth power delivery and ease of maintenance, appealing to both novice and experienced users in the motorcycle community.