SB 1.4.19
catur-hotram karma suddham
prajanam viksya vaidikam
vyadadhad yajña-santatyai
vedam ekam catur-vidham
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
He saw that the sacrifices mentioned in the Vedas were means by which the people’s occupations could be purified. And to simplify the process he divided the one Veda into four, in order to expand them among men.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Formerly there was only the Veda of the name Yajur, and the four divisions of sacrifices were there specifically mentioned. But to make them more easily performable, the Veda was divided into four divisions of sacrifice, just to purify the occupational service of the four orders. Above the four Vedas, namely Rg, Yajur, Sama, and Atharva, there are the Puranas, the Mahabharata, Samhitas, etc., which are known as the fifth Veda. Sri Vyasadeva and his many disciples were all historical personalities, and they were very kind and sympathetic toward the fallen souls of this Age of Kali. As such, the Puranas and Mahabharata were made from related historical facts which explained the teaching of the four Vedas. There is no point in doubting the authority of the Puranas and Mahabharata as parts and parcels of the Vedas. In the Chandogya Upanisad (7.1.4), the Puranas and Mahabharata, generally known as histories, are mentioned as the fifth Veda. According to Srila Jiva Gosvami, that is the way of ascertaining the respective values of the revealed scriptures.