The theme of the Gītā so far is, “Put philosophy into practice,” and “Don’t give up your responsibilities, perform them with wisdom, for the sake of personal evolution.” The central ingredient in these formulae is philosophy and wisdom. So now Krishna will begin explaining how to gain knowledge and become wise.
Chapter 4 begins…
All-Attractive Krishna said: “I explained this eternal wisdom of yoga to the sun-god, who passed it on to the original human being, who passed it on to his son. Passed on like that from one person to the next, it gradually became known to many philosopher-kings, but in the process it also became distorted, degraded and eventually forgotten. Today I will once again explain that ancient science anew, to you.”
Arjuna would ask, “Why me?!” So Krishna says…
“I have chosen you because I can trust you, and you trust me. You are devoted to me, and you are my friend – therefore you can certainly comprehend the ultimate secrets.” [1-3]
To deeply comprehend any topic, we must deeply open our hearts and minds to it. This is not possible without faith and devotion. Therefore, Krishna says that trust and dedication are the foundation of knowledge.
Arjuna asked: “You were born fairly recently. The sun-god was born very long ago. How am I to understand that you were his teacher?” 
This very practical and realistic question shows that the humble faith and devotion of a good disciple does not equate to being gullible or mindless.
All-Attractive Krishna said: “I have passed through many births, and so have you Arjuna. I am aware of all of them, but you are not.” 
Arjuna would ask, “Why can you remember, while I forget?” Krishna answers…
“I only seem to have a body. The truth is that I am never ‘born’ and never ‘age.’ I am the proprietor and master of everything. My ‘body’ is my own manifestation of the power within my own self.” 
We forget things because our memories are accessed via electronic pathways that deteriorate, and are not really under our control or ownership, being only temporarily on loan to us from Mother Nature. Krishna never forgets because he is not in a similar situation: His form is a manifestation of himself, by himself, in himself. Quite unlike the rest of us, there is no difference between the divine energy within Krishna and the divine energy that manifests his specific forms. There is no difference between his ‘body’ and ‘soul.’ This directly contradicts a prevalent Hindu misconception that the divine energy within Krishna is important and the specific form of “Krishna” is not.
Now Arjuna will wonder, “Why do you take the trouble of manifesting yourself within this forgetful world?” So Krishna says…
“I manifest myself whenever the paths of morality become overgrown by the flourishing weeds of immorality. Time after time I manifest to repave the moral paths by protecting those who still walk upon them, and destroying those who do not.” [7-8]