Lord Krishna is instructing Arjuna to destroy all doubts arising from ignorance of the atma or soul and having discrimination between the soul and the body take refuge in karma yoga or the performance of prescribed Vedic activities which is the means to atma tattva or soul realisation.

I bow to the Supreme Lord Krishna, the destroyer of all doubts who has taught the dual faiths of karma yoga and jnana yoga or the cultivation of Vedic knowledge according to the natural attributes and qualifications among humans.

Having explained the means of acquiring spiritual realisation and the disadvantages of achieving spiritual realisation the Supreme Lord Krishna concludes this chapter.

Thus ends the commentaries to Srimad Bhagavad-Gita, Chapter 4 by Madhvacarya..

The sword of knowledge that Lord Krishna is referring to is atma tattva or soul realisation and the yoga or the science of the individual consciousness attaining communion with the ultimate consciousness, He is indicating is karma yoga or the performance of prescribed Vedic activities according to one's rank and station in life.

The sword of spiritual knowledge is the eternal atma or soul. All doubt should be completely uprooted from one's heart and mind and follow the path that Lord Krishna has so concisely explained which is first and foremost to do one's duty in life according to qualification. Following this path one will acquire the spiritual knowledge to cross over samsara the cycle of birth and death and attain moksa liberation from material existence..