A Toyan Flat-Four FS-B400 Engine refers to a small, horizontally opposed four-cylinder engine manufactured by Toyan, essentially a miniature boxer engine with two banks of cylinders lying flat on opposite sides of the crankshaft, often used in hobbyist projects and educational demonstrations due to its design that allows for easy observation of internal workings; Flat-Four signifies the arrangement of the cylinders, while FS-B400 is the specific model designation by Toyan