'dvitīya catur-vyūha' ei--turīya, viśuddha
vāsudeva-the expansion named Vāsudeva; saṅkarṣaṇa-the expansion named Saṅkarṣaṇa; pradyumna-the expansion named Pradyumna; aniruddha-the expansion named Aniruddha; dvitīya catuḥ-vyūha-the second quadruple expansion; ei-this; turīya-transcendental; viśuddha-free from all material contamination.
Vāsudeva, Saṅkarṣaṇa, Pradyumna and Aniruddha constitute this second quadruple. They are purely transcendental.
In his Laghu-bhāgavatāmṛta (Pūrva 5.165-193), Śrīla Rūpa Gosvāmī has refuted the charges directed against the devotees by Śrīpāda Śaṅkarācārya regarding their explanation of the quadruple forms Vāsudeva, Saṅkarṣaṇa, Pradyumna and Aniruddha. He says that these four expansions of Nārāyaṇa are present in the spiritual sky, where They are famous as Mahāvastha. Among Them, Vāsudeva is worshiped within the heart by meditation because He is the predominating Deity of the heart, as explained in Śrīmad-Bhāgavatam (4.3.23).