SB 1.4 - Vyāsa divides the Veda
Prior to kali yuga religious texts were memorized and not recorded. Vyāsa attempted to adapt the Vedic processes to the times. The Vedas require adjustment because to make proper use of the Vedas one is required to know the intricacies of Vedic Sanskrit, which is more difficult than contemporary Sanskrit. In addition the cryptic verses of the Vedas require a qualified guru under whom they can be studied. In kali yuga there are no such gurus. Before undertaking the study of the Vedas, one should have studied the six Vedic corollaries, or limbs called the Vedāṅgas: śikṣā, pronunciation; kalpa, the process of performing sacrifices; vyākaraṇa, grammar; nirukta, the meanings and derivations of difficult words used in the Vedas; jyotiṣa, astronomy and astrology; and chandas, Vedic meters. Who would teach these subjects and who would be capable of learning them in Kali yuga?
Vyāsa saw that the sacrifices mentioned in the Vedas were means by which the occupation of the people could be purified. To simplify the process, he divided the one Veda, Yajur, into four – Ṛg, Yajur, Sāma and Atharva. Seeing the purifying power of Vedic rites performed by the four priests for the people at large who were not inclined for jñāna, yoga or bhakti, Vyāsa divided the one Veda into four for continuation of sacrifice. These sacrifices were accomplished by four priests, the hotā (reciter of Ṛg-veda, offerer of oblations), udgātā (reciter of Sāma-veda, corrector of irregularity), adhvaryu (reciter of Yajur-veda, preparer of items for sacrifice), and the brahmā (reciter of Atharva-veda, knower of all vedas, supervisor). The historical facts (itihāsās) and authentic stories (Purāṇas) are called the fifth Veda. The Chāndogya Upaniṣad (7.1.4) also confirms that the Purāṇas and the itihāsās are the fifth Veda. These explain the teaching of the four Vedas.