The Vishnu Purana is arranged in six amshas and is often cited as the clearest instance of the pancha-lakshana, the five marks by which a Purana is described. It is told as the teaching of Rishi Parashara to Maitreya, moving through creation, the lineages, and the turning of the yuga cycle. Its fifth and longest amsha is given to the life of Krishna, from the years at Gokula and Vrindavan onward, and is read alongside the Bhagavata.
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The Vishnu Purana sets out four yugas of unequal length, running in the ratio 4:3:2:1 — each age shorter than the one before it.
Source: Yuga cycle; Vishnu Purana 1.3; Manusmriti 1.68–71