The Supreme living entity, Krishna, eternally manifests Himself as the catur-vyuha or quadruple plenary expansion.

The supreme living entity, Kṛṣṇa, eternally manifests Himself as the catur-vyūha, or quadruple plenary expansion. Although the Absolute Truth is one without a second, the Absolute Truth displays His unlimited opulences and potencies by expanding Himself in innumerable plenary forms, of which the catur-vyūha is a principal expansion. The original being is Vāsudeva, the Personality of Godhead. When the Godhead manifests His primeval energies and opulences, He is called Saṅkarṣaṇa. Pradyumna is the basis of the Viṣṇu expansion who is the soul of the entire universe, and Aniruddha is the basis of the personal manifestation of Viṣṇu as the Supersoul of every individual entity within the universe. Among the four plenary expansions mentioned here, the original expansion is Vāsudeva, and the other three are considered to be particular manifestations of Him.

Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Eleventh Canto, Chapter 05 – Text 29-30.