Mudgalapurana 4.2.59

4.2.59

śuklāṃ kṛṣṇāṃ caturthīṃ ye kariṣyantyamarādayaḥ |
sarvavratādi bhāveṣu teṣāṃ siddhirbhaviṣyati ||

Those immortals and others who observe the bright and dark fourth lunar days / will attain success in all vows and the like.”