The first chapter demonstrated the nature of Prince Arjuna’s sorrow on the battlefield. He was confused because he believed that he would be miserable irrespective of how he chose to act. At that point, he surrendered to Shri Krishna to guide him.
In the second chapter, Shri Krishna gave the highest Knowledge about the Supreme Reality, explaining elaborately to Arjuna that his sorrow is because of delusion born out of ignorance of his true nature. Shri Krishna reveals that the Self in Arjuna (and every one of us) is the same as the Supreme Reality, Brahman, which is eternal, changeless, and limitless. Experiencing this Knowledge is the goal of spirituality.
At the beginning of the third chapter, Arjuna is confused about why Krishna gave him the highest Knowledge but still insists that he perform his duties as per the doctrine of Karma Yoga. He is not encouraged to pursue the path of Knowledge (Jñana Yoga) to be a man of wisdom as portrayed by Shri Krishna. The initial confusion that Arjuna faced in the first chapter still remained. He was not sure about what action to undertake. Shri Krishna then explains how Karma Yoga is the means for purifying the mind so one can realize the Knowledge of Self through study, reflection, contemplation, and meditation.
The fourth chapter begins with an intent to invoke love and reverence in the heart of Arjuna towards Shri Krishna. It is difficult for a student to accept the ideas being presented by his teacher if there is no love and reverence. Shri Krishna already thought Arjuna was fit to receive this knowledge but wanted to remove lingering doubts in the student’s mind.
Chapter 4, Verse 1
इमं विवस्वते योगं प्रोक्तवानहमव्ययम् ।
विवस्वान्मनवे प्राह मनुरिक्ष्वाकवेऽब्रवीत् ॥१॥
imaṁ vivasvate yogaṁ proktavānahamavyayam,
vivasvānmanave prāha manurikṣvākave’bravīt. (1)
The Lord said: I taught this eternal Yoga to Vivasvan, who passed it on to Manu; and Manu taught it to Ikshvaku.
Vivasvān is the Sun God. Sun is the first to be created, without which no other being can survive. He is an embodiment of selfless work who does his duty precisely without bothering about the praises or curses. Every bit of what the Sun does is for the welfare of all, without any ego or selfish desire. Shri Krishna says that he gave this art of right action to Vivasvān. Vivasvān gave this knowledge to Manu, the first wise man amongst mankind, and Manu taught Ikshvāku, the forefather of Arjuna. In this way, Shri Krishna is glorying this knowledge to be eternal and imperishable and carried forward through generations of kings.
Chapter 4, Verse 2
एवं परम्पराप्राप्तमिमं राजर्षयो विदुः ।
स कालेनेह महता योगो नष्टः परन्तप ॥२॥
evaṁ paramparāprāptamimaṁ rājarṣayo viduḥ,
sa kāleneha mahatā yogo naṣṭaḥ parantapa. (2)
O Parantapa (Scorcher of the foes), the saintly kings thus received this knowledge in a continuous tradition. But with the long passage of time, this Yoga has been lost in this world.
In this way, as a tradition, this knowledge was transferred in succession from Vivasvān to Manu to Ikshvāku to Nimi and so on. All spiritual kings were very familiar with the practice of karma yoga to reach perfection. These kings would hand over the kingdom to their sons after a certain age to retire to the forest to contemplate and meditate.
But this Knowledge was lost in time as everything changes in time. In due course, people were infected by greed, anger, pride, etc. When this knowledge went to people who were not fit students, they couldn’t transfer the teachings in their true essence to future generations. Here Shri Krishna is giving him (and all of us) hope by sharing this knowledge once again in its purest form.
Chapter 4, Verse 3
स एवायं मया तेऽद्य योगः प्रोक्तः पुरातनः ।
भक्तोऽसि मे सखा चेति रहस्यं ह्येतदुत्तमम् ॥३॥
sa evāyaṁ mayā te’dya yogaḥ proktaḥ purātanaḥ,
bhakto’si me sakhā ceti rahasyaṁ hyetaduttamam. (3)
The same ancient Yoga which is the Supreme Secret has been revealed to you today because you are My friend and My devotee.
Shri Krishna says that he is now giving Arjuna the same knowledge (which was lost in time) that he gave to Vivasvān and then passed onto Manu, Ikshvāku, and so on because Arjuna is a dear friend and a devotee of His. Arjuna has totally surrendered to Shri Krishna, with immense love and respect for Him. Additionally, he has a very friendly relationship with Shri Krishna as they grew up together. The ease in their relationship along with Arjuna’s devotion made him a fit student to receive this knowledge from Shri Krishna.
Secret in this context doesn’t mean that it is not to be shared with anyone (like how the pundits have interpreted it). The scriptural study is considered secret because no matter how wise one is in understanding the world, he can never understand the substratum behind the universe on his own until some great master points that out to him.
Chapter 4, Verse 4
अपरं भवतो जन्म परं जन्म विवस्वतः ।
कथमेतद्विजानीयां त्वमादौ प्रोक्तवानिति ॥४॥
aparaṁ bhavato janma paraṁ janma vivasvataḥ,
kathametadvijānīyāṁ tvamādau proktavāniti. (4)
You were born much after Vivasvan. How am I to understand that you taught this Yoga to him in the beginning?
Arjuna doesn’t understand how Shri Krishna who is standing right in front of him and is only a few years older than him could teach Vivasvān, the Sun who was there long before them.
Arjuna is not doubting what Shri Krishna is saying. He is confident that he will get answers to all his questions from his teacher and friend Shri Krishna. He asks Shri Krishna how is he to understand that an individual who is born much later teaches someone who was there prior to his birth.
Shri Krishna in the following verses starts discussing one of the important concepts in Sanātana Dharma, avatāra.