We find a clear reference in Srimad Bhagavatam 1.4.19 that Bhagavan Veda Vyasa divided the Vedas from one into four.
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.
Regarding when this was done [Srimad Bhagavatam 1.4.25] 22 has the answer again. It was done prior to compiling the Mahabharata, which means in the Dwapara Yuga before Kali Yuga since the sequence is mentioned in the verses.
Out of compassion, the great sage thought it wise that this would enable men to achieve the ultimate goal of life. Thus he compiled the great historical narration called the Mahābhārata for women, laborers and friends of the twice-born.
We also find in Srimad Bhagavatam 12.6.50,
Śrīla Vyāsadeva separated the mantras of the Ṛg, Atharva, Yajur and Sāma Vedas into four divisions, just as one sorts out a mixed collection of jewels into piles. Thus he composed four distinct Vedic literatures.