Śatapatha Brāhmaṇa ("of a hundred paths") 1.9.2.6

1.9.2.6

catasro devatā yajati | catasro vai mithunaṃ dvandvaṃ vai mithunaṃ dve dve hi khalu bhavato mithunamevaitatprajanane kriyate tasmāccatasro devatā yajati

He makes offering to four deities. Four doubtless means a couple; for a couple means a pair (dvandva, lit. two and two), and two and two indeed they are: thus a productive union is thereby effected; and accordingly he makes offering to four deities.