Sri Caitanya Mahāprabhu and the Acintya-bhedābheda-tattva (inconceivable oneness and difference)
Sri Caitanya Mahāprabhu took two principles from each of the four Vaishnava sampradāyas and incorporated them into his own sampradāya, revealing thus the Vaishnava philosophy in its fullness.
« Vedānta-sūtra: The Govinda-bhāṣya of Baladeva Vidyābhūṣaṇa
Sri Caitanya Mahāprabhu: Acintya-bhedābheda-tattva (inconceivable oneness and difference)
Both Rāmānujācārya and Madhvācārya had to adjust their presentations to what could be understood by their students. Apart from them, there are also Viṣṇusvāmi and Nimbārka, the founder-ācāryas of the other two Vaishnava sampradāyas, namely the Viṣṇusvāmi-sampradāya (later the Baladeva-sampradāya) and the Nimbārka-sampradāya. Sri Caitanya Mahāprabhu built on top of the foundation created by them, giving the ultimate conclusion of the Vedas.
In his Sri Navadvīpa Dhāma Māhātmya, Srila Bhaktivinoda Ṭhākura explains that Sri Caitanya Mahāprabhu took two principles from each of the four Vaishnava sampradāyas and incorporated them into his own sampradāya, revealing thus the Vaishnava philosophy in its fullness. It's also mentioned how the four ācāryas took birth as devotees during the pastimes of Lord Caitanya, joining forces to spread his movement.
The philosophy of Viṣṇusvāmi is called Suddhādvaita-vāda (purified monism) and his commentary on the Vedanta-sutra is the Sarvajña Bhāṣya. This is the oldest of the four commentaries. It explains that the souls are parts and parcels of the absolute truth, one with Him, but at the same time separated, just like sparks coming from a fire. The material world is manifested from the energy of the Lord and is thus not false. Just like bars of gold can be molded into different ornaments and later melted back into bars, the material world is created by the molding of the Lord's eternal energy. The Lord has no material qualities, but at the same time is full of eternity, knowledge, and bliss. He is omniscient, omnipotent, Supreme, the bestower of the fruits of action, and the cause of all auspiciousness. The Lord can never fall under the influence of Maya, but being very small, the jīvas can. When they do so, they forget about the Supreme Lord and are covered by ignorance. The goal of life is to attain uninterrupted contact with the Lord in Vaikuṇṭha, from where there is no return. This supreme goal can only be achieved through the practice of devotional service.
Although the names sound almost the same, the philosophy of Suddhādvaita of Viṣṇusvāmi is different from the Śuddha-dvaita of Madhvācārya. The philosophy of Viṣṇusvāmi is purified monism, and the name comes from the words Śuddha Advaita and the philosophy of Madhvācārya is purified dualism, and the name comes from the words Śuddha Dvaita. Due to the rules of the Sanskrit language, Śuddha Advaita is written as Śuddhādvaita and Śuddha Dvaita as Śuddha-dvaita. The word "vāda" means simply "doctrine", or "philosophy", so we can say Śuddha-dvaita-vāda, Śuddha-dvaitavāda, or simply Śuddha-dvaita philosophy; the meaning is the same.
The philosophy of Nimbārka is called Dvaitādvaita-vāda (monism and dualism, oneness and difference), and sits between the non-dualism of Śaṅkarācārya and the purified dualism of Madhvācārya. The philosophy of Nimbārka is described in the Parijāta Saurabha Bhāṣya, his commentary on the Vedanta-sutra. According to him, the concepts of unity and differentiation between the Lord's energies are equally true, just like waves and the ocean or the sun and its rays. Both matter and souls are considered parts of God, although at the same time separated from him.
According to him, the cause of bondage for the soul is contact with karma (material activities), which in turn results from ignorance, and the process of salvation is through the uninterrupted worship of the Lord. The ultimate goal is the realization of the eternal nature of the soul. The most important characteristic, however, is the worship of Rādhā and Krsna in a svakīya mood (conjugal love in marriage), with emphasis on sambhoga (union).
In this way, all four Vaishnava ācāryas wrote their commentaries on the Vedanta-sutra, explaining their philosophical views. Although different in certain points, they all describe how the soul is, on the one hand, one with Krsna and, on the other hand, different from him, and thus the basic conclusion is the same. The soul is one with Krsna because is part of his spiritual potency, but at the same time, the soul is eternally an individual, who can't ever merge back into Krsna, although the soul can be temporarily put into His effulgence. This is the basis for the concepts of devotional service and spiritual rasas since there is no question of relationships with oneself. Relationship implies two, and therefore, the distinction between the Lord and the soul is essential to establish the eternal relationship between them.
Srila Prabhupada mentions the four Vaishnava commentaries on the Vedanta-sutras in his purport to CC Antya-līlā 2.95:
"The philosophers known as kevalādvaita-vādīs generally occupy themselves with hearing the Śārīraka-bhāṣya, a commentary by Śaṅkarācārya advocating that one impersonally consider oneself the Supreme Lord. Such Māyāvāda philosophical commentaries upon the Vedānta-sūtra are simply imaginary, but there are other commentaries on the Vedānta-sūtra. The commentary by Śrīla Rāmānujācārya, known as Śrī-bhāṣya, establishes the viśiṣṭādvaita-vāda philosophy. Similarly, in the Brahma-sampradāya, Madhvācārya’s Pūrṇaprajña-bhāṣya establishes śuddha-dvaita-vāda. In the Kumāra-sampradāya, or Nimbārka-sampradāya, Śrī Nimbārka establishes the philosophy of dvaitādvaita-vāda in the Pārijāta-saurabha-bhāṣya. And in the Viṣṇu-svāmi-sampradāya, or Rudra-sampradāya, which comes from Lord Śiva, Viṣṇu Svāmī has written a commentary called Sarvajñabhāṣya, which establishes śuddhādvaita-vāda.
A Vaiṣṇava should study the commentaries on the Vedānta-sūtra written by the four sampradāya-ācāryas, namely Śrī Rāmānujācārya, Madhvācārya, Viṣṇu Svāmī and Nimbārka, for these commentaries are based upon the philosophy that the Lord is the master and that all living entities are His eternal servants. One interested in studying Vedānta philosophy properly must study these commentaries, especially if he is a Vaiṣṇava. These commentaries are always adored by Vaiṣṇavas. The commentary by Śrīla Bhaktisiddhānta Sarasvatī is elaborately given in the Ādi-līlā, chapter seven, text 101. The Māyāvāda commentary Śārīraka-bhāṣya is like poison for a Vaiṣṇava. It should not be touched at all. Śrīla Bhaktivinoda Ṭhākura remarks that even a mahā-bhāgavata, or highly elevated devotee who has surrendered himself unto the lotus feet of Kṛṣṇa, sometimes falls down from pure devotional service if he hears the Māyāvāda philosophy of the Śārīraka-bhāṣya. This commentary should therefore be shunned by all Vaiṣṇavas."
« Vedānta-sūtra: The Govinda-bhāṣya of Baladeva Vidyābhūṣaṇa