Bhagavatam says- tasmād guruṁ prapadyeta jijñāsuḥ śreya uttamam
śābde pare ca niṣṇātaṁ brahmaṇy upaśamāśrayam
‘Therefore, one desirous of knowing the ultimate welfare should approach a preceptor who is well versed in the Vedas, who has realized the Absolute Reality, and who has thus become the abode of peace.’ (SB 11.3.21)” What do scriptures say about this?
- Let him live forty-eight years as a student for the (four) Vedas,
- Or twelve years for each Veda,
- Or until he has learnt it.
- If he behaves thus, his fame when he has become a Snātaka (i.e. when he has taken the bath at the end of his studentship) will be (such that people will say of him), 'To-day he stays there; to-day he stays there.'
- There are three (kinds of) Snātakas: a Vidyā-snātaka (i.e. a Snātaka by knowledge), a Vrata-snātaka (i.e. a Snātaka by the completion of his vows), and a Vidyā-vrata-snātaka (i.e. a Snātaka both by knowledge and by the completion of his vows).
- He who performs the Samāvartana ceremony, after having finished the study of the Veda, but before the time of his vows has expired, is a Vidyā-snātaka.
A snataka later becomes a Guru. As you can see you can study four Vedas or one Veda. So it is not mandatory to study all four vedas. One Veda is also fine.
I am posting a discussion on the quality of a Guru given in Linga Purana. Only that person who has a decisive knowledge of the Atman can be a Guru. Knowledge of scripture without direct experience of the Atman will not be of much use.
Quality of a Guru
The preceptor who is conversant with the scriptural texts, who is intelligent, who is favorably disposed towards common people, who performs penance, who is engaged in following accepted worldly conventions and who knows the principles is known as one who bestows salvation.
He may be endowed with all characteristic qualities; he may be an expert in all sacred texts, and he may be conversant with all means and modes, but if he is devoid of the real truth, all the other qualities shall be futile to him.
If he has no decisive knowledge of the Atman which is self-evident great principle, he is not blessed himself. How can he bless others?
The brahmin who is enlightened is pure and is capable of accomplishing things. But whence can there be enlightenment in one who is devoid of principles? Whence can there be comprehension of self? …………… Hence, those liberated souls who are conversant with the principles (realities) can redeem others. The principle that produces perfect cognition originates from his greatest bliss.
Only he who has understood the truth can be a guide unto bliss and not one who is a preceptor only in name but who is devoid of cognition.
Such people cannot redeem one another. Can a rock redeem another rock? Those who have perfect cognition only in name can have salvation only in name.
Command that causes the destruction of bonds is instantaneously born by the sight and touch of the yogins or even by conversing with them.
Or, the preceptor may enter the physical body of the disciple by the yogic path and enlighten him by means of the yogic power after understanding all the entities.
Linga Purana II.20.34-43
Your quoted original Sanskrit shloka didn't mention Veda explicitly. So it's not mandatory in outward sense of learning.
It seems to be added by the translator as commentary.
Translator insertions without using brackets is very common these days, so you have to develop a basic grip on sanskrit.
