Mach 4 refers to a speed that is four times the speed of sound in air. The speed of sound, known as Mach 1, varies with temperature and altitude but is approximately 343 meters per second (1,125 feet per second) at sea level. Therefore, Mach 4 is about 1,372 meters per second (4,500 feet per second). This speed is typically achieved by advanced aircraft and missiles, often designed for military applications. At such high velocities, the effects of air resistance and heat become significant, requiring specialized materials and engineering to ensure safety and performance.