I found from a post (see question no.5) , Brahma is the priest in Atharva Veda. As all 4 Vedas are given by Brahma, then who are the priests of remaining 3 Vedas?
As Vedas are for knowledge then, What is the need of a priest in Vedas?
In his book Indian Philosophy: A Critical Survey, Chandradhar Sharma writes (p 14), available here - https://archive.org/details/IndianPhilosophyACriticalSurvey :
The name 'Veda' (knowledge) stands for the Mantras and the Brahmanas (mantra-brahmanayor veda-namadheyam). Mantra means aa hymn addressed to some god or goddess. The collection of the Mantras is called 'Samhita'. There are four Samhitas--Rk, Sama, Yajuh and Atharva. These are said to be compiled for the smooth performance of the Vedic sacrifices. A Vedic sacrifice needs four main priests--Hota, who addresses hymns in praise of the gods to invoke their presence and participation in the sacrifices; Udgata, 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 Samhitas are said to be compiled to fulfil the needs of the four main priests--Rk for the Hota, Sama for the Udgata, Yajuh for the Adharyu and Atharva for the Brahma.
Each one of the 4 Vedas is generally arranged into a 'Work' portion which contains the Samhitas, Brahmanas and Aranyakas; and a 'Knowledge' portion which contains the Upanishads. They are meant to be a guide for the entire life of a person. Priests are needed for the Work portion, they are not needed for the Knowledge - of Brahman - portion.
So why do the Vedas, meaning the karma-kanda portion, teach these rituals? It is not that these rituals are inherently bad or good, the Vedas teach men those things which will bring them to the four goals or ideals of life – dharma, artha, kama, and moksha. Swami Nikhilananda writes in his Introduction to his book Self-Knowledge (a translation of Shankaracharya’s Atmabodha), pp 21-2:
…Dharma is righteousness; it is the law of inner growth and the basis of man’s actions. It is in harmony with man’s spiritual evolution. Therefore by following dharma one attains success in all action. By negating dharma one brings confusion into one’s life and retards the clock of progress. Dharma is not a sort of duty imposed from outside, but a sense of righteousness, integrity, and honour with which one is born as a result of one’s past actions. So every man has his own dharma, in consequence of which he reacts in his own unique way to the outside world. His education and environment give to this basic life-form only an outer shape. By fulfilling his dharma a man marches along the path of progress until he attains the supreme dharma of all beings, namely, the realization of Truth.
Artha, or wealth, is a legitimate goal of pursuit at a certain stage of man’s life. It is, with most people, an effective mode of self-expression and an important means of establishing fellowship with others. But wealth must be acquired by according to dharma, righteousness; otherwise, instead of serving a spiritual purpose, it will aggravate greed and lust for power and ultimately be a cause of misery.
Kama is the fulfillment of sensuous and esthetic desire. Craving for sense pleasure is present in many sensitive persons to whom enjoyment of wealth appears gross and therefore inadequate. But Kama, too, must be guided by dharma; otherwise it degenerates into voluptuousness.
The satisfaction derived from the pursuit of dharma, artha, and kama is neither deep nor abiding. There remains a hunger of the soul that can be fulfilled only by the attainment of moksha, or Freedom. The first three ideals belong to the material world, and the happiness derived from them is therefore ephemeral and illusory. But the ideal of Freedom can be realized only in the realm of Spirit, and the Bliss that follows is everlasting. Therefore the realization of moksha, Freedom, is the coping-stone of human life; and the pursuit of righteousness, wealth, and esthetic satisfaction only support it.
The karma-kanda is meant to teach man those things necessary for the attainment of dharma, artha, and kama; the jnana-kanda, the Upanishads, are meant to teach man those things necessary to attain moksha. To follow ONLY those portions of the Vedas to attain only artha and kama are bad to one’s eventual karma; the following of all the Vedas and through them attain all four ideals, is the real purport of the Vedas and are meant to guide man through all the stages of life.
In his translation of the Mahanarayana Upanishad, Swami Vimalananda writes in his commentary to verse I.53 (p 72):
The Vedas are meant to generate in those who follow them certain potencies which help in all situations and conditions. The highest ideal held forth by the Vedas to those who have completed the discipline prescribed in them is realization of the Infinite Self dwelling in all creatures as the reality behind their finite existences [Gita 7.30, 6.30]. Those who attained to the goal have no friends or foes. They see equally God in all [Gita 5.18-19]. They are extremely rare. Therefore the Vedas have only a luminously precious fraction describing them. The largest part of the Vedas is meant for the common man who is tossed by likes and dislikes. Failure to recognize the fears and hopes of the natural man make a scripture unrealistic. Ideals unconnected with the practical needs of human nature cannot enter into the daily life of the ordinary man. To suppose that the Vedas always deal with high philosophy and ethics only will be a delusion. Mystical formulas like the present one [this verse] are found on many passages of the Vedas. They are given to propitiate friendly agencies and to suppress the enemies. Without peace and prosperity, santi and pusti, higher religious aspiration is impossible. That is why we find in the Vedas prayers like this one [verse] which deal with the lower purusarthas.
The breakdown of the Vedas (including into the Upanishads) is arbitrary, done by the sage Vyasa. The breakdown of the Vedas into a karma kanda portion and a jnana kanda portion of the Vedas is arbitrary - but has been recognized for thousands of years by innumerable commentators of all sects.