the four classical chemical elements (fire, earth, air and water)
the four great civilizations of the world (China, Babylon, India and Egypt; as put forward by Chinese scholar Liang Qichao in 1900)
the four most important tournaments in a particular sport; grand slam tournaments; majors
the four major pollution-caused illnesses (Minamata disease, Niigata Minamata disease, Yokkaichi asthma, Itai-itai disease); four big pollution diseases of Japan
the four great Japanese grammar texts (Yamada, Matsushita, Hashimoto and Tokieda)
Four Great Inventions (of China: the compass, gunpowder, papermaking and printing)