Why Gautama, the Buddha said about 3 Vedas only? [duplicate]

Question

The Sutta Nipāta 1026 (P.303) mentions about 3 vedas only.

His age is a hundred and twenty years, by clan he is a Bāvarī, upon his body appear three signs, Three Vedas he has mastered all.

We should remember that that Gautama, the Buddha took birth in a Hindu family only. He studied the prevalent literature , observed the prevalent customs and practices , got vexed with the degraded form of spiritual aspects in his times, adopted his own method and finally reached the stage SAT, which Veda described in RV 1.164.46.

Can we infer that by the time Gautama, the Buddha got SELF REALISATION, there were only 3 Vedas? If so, when was the 4th Veda,ie., Atharva Veda added to the list?

Does the statement of Buddha - His age is a hundred and twenty years - indicate the normal life span of a humans, as 120 years, at that point of time?

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Edit 25-11-19

No, the question Why are Vedas often referred to as “three Vedas” instead of four? Why is Atharva Veda excluded from this list?, does not contain a reference to sayings to Gautama, a Hindu Prince and who attained SELF REALISATION, at a later date. Hence, this question of mine is not a duplicate. Further, the answers thereto do not satisfy me.

Answer 1

There are four vedas, although sometimes the 3 main vedas are referred to. Chandradhar Sharma writes in his book A Critical Survey of Indian Philosophy p2-3 (https://archive.org/details/IndianPhilosophyACriticalSurvey):

The name ‘Veda* (knowledge) stands for the Mantras and the Brâhmanas (mantra-brähmanayor veda-nämadheyam). Mantra means a hymn addressed to some god or goddess. The collection of the Mantras is called ‘Samhitä*. There are four Samhitäs— Rk, Säma, Yajuh and Atharva. These are said to be compiled for the smooth performance of the Vedic sacrifices. A Vedic sacrifice needs four main priests— Hotä, who addresses hymns in praise of the gods to invoke their presence and participation in the sacrifice; Udgätä, who sings the hymns in sweet musical tones to entertain and please the gods; Adhvaryu, who performs the sacrifice according to the strict ritualistic code and gives offerings to the gods; and Brahma, who is the general supervisor well-versed in all the Vedas. The four Samhitäs are said to be compiled to fulfil the needs of these four main priests— Rk for the Hotä, Säma for the Udgätä, Yajuh for the Adhvaryu and Atharva for the Brahmä. Sometimes the Vedas are referred to only as ‘Trayï,* omitting the Atharva. Rk means a verse, Säma means a song; Yajuh means a prose passage. Thus we see that the Samhitä-bhäga or the Mantra-portion of the Veda is the Hymnology addressed to the various gods and goddesses. Rk-Samhitä is regarded as the oldest and also the most important. The Rsis of the Vedas are not the authors, but only the ‘seers’ of the Mantras (rsayo mantra-drastärah). The Brâhmanas, unlike the Mantras, are written in prose. They are the elaboration of the complicated ritualism of the Vedas. They deal with the rules and regulations laid down for the performance of the rites and the sacrifices. Their name ‘Brähmana* is derived from the word ‘Brahman* which originally means a prayer. There is little philosophy in these, though some philosophical ideas flash here and there in the course of some speculative digressions. The appendages to these Brâhmanas are called Aranyakas mainly because they were composed in the calmness of the forests. The Aranyakas mark the transition from the ritualistic to the philosophic thought. We find here a mystic interpretation of the Vedic sacrifices. The concluding portions of the Aranyakas are called the Upanisads. These are intensely philosophical and spiritual and may be rightly regarded as the cream of the Vedic philosophy. The Mantras and the Brâhmanas are called the Karma-Kända or the portion dealing with the sacrificial actions, and the Aranyakas and the Upanisads are called the Jnanä-Kända or the portion dealing with knowledge. Some people include the Aranyakas in the Karma-Kärida. Really speaking, they represent a transition from the Karma-Kända to the Jnanä-Kända. The Upanisads are also known as ‘Vedänta* or ‘the end of the Veda*, firstly because they are literally the concluding portion, the end, of the Vedas, and secondly because they are the essence, the cream, the height, of the Vedic philosophy.