In the Vedas and earliest brāhmaṇas, we find evidence only for the brahmacārin and gṛhastha āśramas as a system followed by common people. However, some form of sannyāsa did exist, so did vānaprastha, both of which were mostly a sagely affair before the sage gets enlightened. It is this asceticism and celibacy of Agastya which his wife Lopāmudrā intends to stop (in Rigveda) by enlightening him about the duty of a married couple, after which Agastya is enlightened.
For example, works in the post-Vedic period see Vedic sages as having followed both śramaṇa and gṛhastha lifestyles. In Rigveda, we already find the Keśī sūkta about the Keśī munis who have left everything behind but their mortal body. Taittirīya Āraṇyaka glorifies them as the first sages and first śramaṇas, still knowing no dichotomy between brāhmaṇa and śramaṇa. Sages who didn’t yet obtain their vision should have practised austerities being munis before they become inspired. Indra is said to be the lord of those who wander, seeking the truth. The munis wander, following the track of the Cow (the inspiration) and after they find it, they get their share of soma.
The concept of four āśramas as a social ideal (just like the four varṇas) for all people is not actually implied in Vedas or in the śrauta/gṛhya (smārta) karmas. Śramaṇa lifestyle became a common phenomenon after the success of some śramaṇa schools like Ājīvikas, Bauddhas and Jains, and soon people started escaping their normal course of life into forests to “find their self/non-self”. This became the case after sagehood ended and ideas like saṃsāra or endless rebirths emerged. A glorification of monkhood emerged, and people began rushing to forests allured by certain śramaṇa traditions. Dharmasūtras do approve of śramaṇa lifestyle, but only after one has “paid off his debts” by living as a brahmacārī and gṛhastha, learning Vedas and doing the saṃskāras without any omission. Brahmanism allowed for śramaṇa lifestyle within its own authoritative śramaṇaka sūtras, which prescribe the laws and rules to be followed in forest. Brahmin mendicants are called parivrājakas, and already feature in epics. Still, early dharmasūtras, fearing śramaṇa labels by mīmāṃsakas, are defensive about gṛhastha āśrama and maintain that even if one doesn’t follow sannyāsa or vānaprastha and just follows the gṛhastha lifestyle, he is still entitled to “liberation”.