The fourth chapter of the Bhagavad Gita is Jnana Karma Sanyasa Yoga. In this chapter, Krishna glorifies the Karma Yoga and imparts the Transcendental Knowledge (the knowledge of the soul and the Ultimate Truth) to Arjuna. He reveals the reason behind his appearance in this material world. He reveals that even though he is eternal, he reincarnates time after time to re-establish dharma and peace on this Earth. His births and activities are eternal and are never contaminated by material flaws. Those persons who know and understand this Truth engage in his devotion with full faith and eventually attain Him. They do not have to take birth in this world again.
The Supreme Lord Shree Krishna said: I taught this eternal science of Yog to the Sun-god, Vivasvan, who passed it on to Manu; and Manu in turn instructed it to Ikshvaku.OPEN VERSE
O subduer of enemies, the saintly kings thus received this science of Yog in a continuous tradition. But with the long passage of time, it was lost to the world.OPEN VERSE
The same ancient knowledge of Yog, which is the supreme secret, I am today revealing unto you, because you are my friend as well as my devotee, who can understand this transcendental wisdom.OPEN VERSE
Arjun said: You were born much after Vivasvan. How am I to understand that in the beginning you instructed this science to him?OPEN VERSE
The Supreme Lord said: Both you and I have had many births, O Arjun. You have forgotten them, while I remember them all, O Parantapa.OPEN VERSE