"Vamana, Brahma, Ananta Sesha and Sanaka Kumara will appear as brahmanas by the order of Visnu, for the preservation of eternal righteousness in kali yuga. Visnuswami, Madhvacarya, Ramanuja and Nimbaditya will appear respectively as a portion of Vamana, Brahma, Ananta Sesha and Sanaka Kumara. These four saviours will be the establishers of the four authorised and empowered spiritual channels of disciplic succession in the period calculated from the reign of King Vikrama in 54 B.C. subsequently through the 432,000 year cycle of kali yuga. These four authorised and empowered spiritual channels of disciplic succession are to be fully accepted by all beings; as any word, combination of words or formulation of sound frequencies, invoked or addressed, audible or inaudible, secret or revealed, ancient or contemporary outside their auspices prove to have absolutely no efficacy."
Garga Samhita, 10:61:23-26
"There are four Sampradayas from the beginning of the creation. One is called Brahma Sampradaya, and is coming down by disciplic succession from Brahma; another Sampradaya is coming down from Laksmi, called Sri Sampradaya; another is coming down from the Kumaras, they are known as Nimbarka Sampradaya; another Sampradaya is coming from Lord Siva, Rudra Sampradaya or Visnu Swami. These are four bona fide Sampradayas that are accepted by the bona fide spiritualists. The Impersonalist Sampradaya is not original, neither the Impersonalist Sampradaya or party can help us. At the present moment there are so many Sampradayas, but we have to test them about their method of disciplic understanding. Anyway, all the four Sampradayas above mentioned, they are after worshipping the Supreme Lord Visnu, in His different Expansions, and some of them are in favor of worshipping Radha Krishna."
Srila Prabhupada Letter to Upendra, 02-13-68, Boston
"There are four sects of worshipful devotees of the Lord, and the chief amongst them are the Brahma-sampradaya, Rudra-sampradaya and Sri-sampradaya, descending directly from Lord Brahma, Lord Siva and the goddess of fortune, Laksmi, respectively. Besides the above-mentioned three sampradayas, there is the Kumara-sampradaya, descending from Sanat-kumara. All of the four original sampradayas are still scrupulously engaged in the transcendental service of the Lord up to date, and they all declare that Lord Krsna, Mukunda, is the Supreme Personality of Godhead, and no other personality is equal to Him or greater than Him."
Srimad-Bhagavatam 1:18:21 Purport
"Therefore there is no difference of conclusions amongst the Vaisnava acaryas regarding the Lord and the devotees. Lord Caitanya asserts that the living entity (jiva) is eternally the servitor of the Lord and that he is simultaneously one with and different from the Lord. This tattva of Lord Caitanya's is shared by all four sampradayas of the Vaisnava school (all accepting eternal servitude to the Lord even after salvation), and there is no authorized Vaisnava acarya who may think of the Lord and himself as one."
"In the Vaisnava-sampradaya, some devotees worship Radha-Krsna, and others worship Sita-Rama and Laksmi-Narayana. Some also worship Rukmini-Krsna. All of these are the same, and all of the devotees are Vaisnavas. Whether one chants Hare Krsna or Hare Rama, it is not very important. Worship of the demigods, however, is not recommended."
Teachings of Lord Kapila, Verse 44
"The philosophers known as kevaladvaita-vadis generally occupy themselves with hearing the Sariraka-bhasya, a commentary by Sankaracarya advocating that one impersonally consider oneself the Supreme Lord. Such Mayavada philosophical commentaries upon Vedanta are simply imaginary, but there are other commentaries on Vedanta philosophy. The commentary by Srila Ramanujacarya, known as Sri-bhasya, establishes the visistadvaita-vada philosophy. Similarly, in the Brahma-sampradaya, Madhvacarya's Purnaprajna-bhasya establishes suddha-dvaita-vada. In the Kumara-sampradaya, or Nimbarka-sampradaya, Sri Nimbarka establishes the philosophy of dvaitadvaita-vada in the Parijata-saurabha-bhasya. And in the Visnusvami-sampradaya, or Rudra-sampradaya, which comes from Lord Siva, Visnusvami has written a commentary called Sarvajna-bhasya, which establishes suddhadvaita-vada.
A Vaisnava should study the commentaries on Vedanta-sutra written by the four sampradaya-acaryas, namely Sri Ramanujacarya, Madhvacarya, Visnusvami and Nimbarka, 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 Vedanta philosophy properly must study these commentaries, especially if he is a Vaisnava. These commentaries are always adored by Vaisnavas."
Caitanya-caritamrta, Antya lila 2:95
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.