Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.2.11

8.3.2.11

yadveva catasraḥ | caturakṣaraṃ vā antarikṣaṃ dvyakṣarā itarāścitayastadyāvadantarikṣaṃ tāvattatkṛtvopadadhāti

And, again, why there are four,--'antariksha' (air) consists of four syllables, and the other layers (kiti) consist of two syllables; hence as much as the air consists of, so much he makes it in laying it down.