Shatapatha Brahmana 1.2.4.21

1.2.4.21

tūṣṇīṃ caturtham | sa yadimāṃllokānati caturthamasti vā na vā tenaivaitaddviṣantam bhrātṛvyamavabādhate 'naddhā vai tadyadimāṃllokānati caturthamasti vā na vānaddho tadyattūṣṇīṃ tasmāttūṣṇīṃ caturtham

Silently (he throws) a fourth time. What fourth world there may or may not be beyond these (three), by that one he thereby drives away the spiteful enemy. For uncertain indeed is what fourth world there may or may not be beyond these (three); and uncertain also is what (is done) silently: for that reason (he throws) silently a fourth time.