The fourth chapter opens with a claim about transmission. This teaching, Krishna says, was given to Vivasvat the sun, who gave it to Manu, who gave it to Ikshvaku, and so it passed down a line of royal seers until long lapse of time destroyed it. Arjuna raises the obvious problem: Vivasvat lived long ago, and Krishna was born recently, so how could he have taught him? The answer is the passage that turns a philosophical dialogue into a theophany in waiting.
Both of them have passed through many births, Krishna says, and he remembers them all while Arjuna does not. Though unborn and imperishable, he takes on birth by his own power, and does so age after age: whenever right order weakens and disorder rises, he brings himself into being, to protect the good, to destroy those who do harm, and to establish right order again. Those verses are the classical scriptural basis for the doctrine of divine descent, and they are quoted more often than any other passage in the chapter.
What follows draws the practical consequence. Anyone who truly knows the divine birth and work is not born again but comes to Krishna. Freed from attachment, fear and anger, absorbed in him, many have been purified by the austerity of knowledge. And whichever way people approach him, he meets them that way, since all paths lead to him. That last verse has done heavy service in modern arguments for religious pluralism, and it does say what it is taken to say, though its immediate context is a statement about worship reaching its object rather than a survey of the world's religions.