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 great Japanese grammar texts (Yamada, Matsushita, Hashimoto and Tokieda)
the four major pollution-caused illnesses (Minamata disease, Niigata Minamata disease, Yokkaichi asthma, Itai-itai disease); four big pollution diseases of Japan