Jaiminīya Upaniṣad Brāhmaṇa 2.6.4

2.6.4

sa yadi brūyāc caturo ma āgāyeti prāṇa udgītha ity eva vidvāṁś caturo manasā dhyāyet
catvāro hi prāṇo 'pāno vyānas samānaḥ
catvāro haivāsyājāyante /

If he should say: “Chant for me for four”, knowing that breath is the udgītha, he should mentally focus on four.
For breath, out-breath, diffused breath, and equalized breath are four.
Four indeed are born to him.