Kesava Kasmiri's Commentary
The third chapter established that one desiring moksa or liberation from the cycle of birth and death, whose desires for sense gratification had not been extinguished should engage themselves in karma yoga or performance of prescribed Vedic activities assisted by some Vedic knowledge and feeling completely free from the fear of falling from ones position. Everyone is not qualified for jnana yoga or the cultivation of Vedic wisdom and even those who are qualified should still follow the path of karma yoga performing prescribed Vedic activities as a matter of duty as an example to the common man for the maintenance of world order.
Now in the fourth chapter Lord Krishna explains karma yoga in the light of historical tradition and that by instructing the knowledge of yoga or the science of the individual consciousness attaining communion with the ultimate consciousness, He Himself performed karma yoga in giving these instructions in the past to Visvavan the demigod of the sun and now to His friend Arjuna because he is His surrendered disciple. It should not be thought that Lord Krishna is speaking it just to motivate Arjuna to battle or that the science of this eternal yoga is recent because it was spoken before 120 million years previously at the commencement of this time period of Manu in order to protect all the worlds. Lord Krishna taught this imperishable yoga for the highest benefit of the human species and for the preservation of universal order. Visvavan after gave it to his son Satyavrata who was the Manu of that time period and he later gave it to his eldest son Iksvaku.