Shatapatha Brahmana 11.1.2.8

11.1.2.8

atho api catasro'tiriktāḥ syuḥ tadyathaikaivaṃ catasrastrayo vā ime lokāstadimāneva lokāṃstisṛbhirāpnoti prajāpatirvā atīmāṃlokāṃścaturthastatprajāpatimeva caturthyāpnoti saiṣā sampatsa yajñaḥ

And there may be four additional ones,--as the one so the four. There are these three worlds; these worlds he thus gains by three (oblations); and Pragâpati, indeed, is the fourth beyond these worlds: by the fourth (oblation) he thus gains Pragâpati,--this is that perfection, the sacrifice.