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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
चातुर्होत्रं कर्म शुद्धं प्रजानां वीक्ष्य वैदिकम् । व्यदधाद्यज्ञसन्तत्यै वेदमेकं चतुर्विधम्
cāturhotraṃ karma śuddhaṃ prajānāṃ vīkṣya vaidikam | vyadadhādyajñasantatyai vedamekaṃ caturvidham
SB 1.4.19