Just like when earth rotates on its axis a day is formed consisting of four parts: morning- noon- evening and the night. After every night comes the morning or when its axis is tilted, seasons come into play : spring-summer-autumn-and winter and after every winter comes the spring.

This division of  days or seasons is cyclic and  is autonomous in nature. Similarly, the world drama is divided into four parts: Golden age, silver age, copper and iron age. it is repeated after every spin of its cycle.

 According to some philosophical books of Hinduism and also Spirituality, states that  the cycle of one span of the world drama is  about 5000 years. Where each cycle is divided into four parts of 1250 years each.