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 Blessed Lord said: I imparted this imperishable Yoga to Vivasvan(Sun-God), Vivasvan taught this to Manu, and Manu transmitted this to Iksavaku.OPEN VERSE
The king-sages knew this (yoga) which was received thus in regular succession. That Yoga, O destroyer of foes, in now lost owing to a long lapse of time.OPEN VERSE
That ancient Yoga itself, which is this, has been taught to you by Me today, considering that you are My devotee and friend, For, this (Yoga) is a profound secret.OPEN VERSE
Arjuna said: Your birth was later, (whereas) the birth of Vivasvan(Sun-God) was earlier. How am I to understand this that You instructed (him) in the beginning?OPEN VERSE
The Blessed Lord said: O Arjuna, many lives of Mine have passed, and so have yours. I know them all, (but) you know not, O scorcher of enemies!OPEN VERSE