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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
ऋगथर्वयजुःसाम्नां राशीरुद्धृत्य वर्गशः । चतस्रः संहिताश्चक्रे मन्त्रैर्मणिगणा इव
ṛgatharvayajuḥsāmnāṃ rāśīruddhṛtya vargaśaḥ | catasraḥ saṃhitāścakre mantrairmaṇigaṇā iva
SB 12.6.50