As we saw in Chapter 3, Lord Krishna advices Arjuna to fight in full knowledge of the Supreme. In the 4th chapter he moves on to explain what this transcendental knowledge is.
If sacrifice of one's material possessions is not directed towards spiritual realization, then such sacrifice becomes simply material. But one who performs such sacrifices with a purely spiritual objective, or in devotional service, that is perfect sacrifice.
Krishna glorifies transcendental knowledge and requests Arjuna to arm himself with this knowledge which burns all sinful reactions to ashes and fight. Arjuna is confused; he asks the Lord whether the renunciation of work or work in devotion is superior.
One who neither hates nor desires the fruits of his activities is known to be always renounced. Thus, outwardly performing all actions but inwardly renouncing their fruits, wise men, purified by the fire of transcendental knowledge, attain peace, detachment, forbearance, spiritual vision & bliss.
The Lord proceeds to explain how one can detach himself from the fruits of his activities. He states that when one stands fast in Krishna consciousness, and neither grieves nor rejoices upon achieving something unpleasant or pleasant reaches the transcendental stage. One should abandon all material desires and engage his body, mind & soul in the service of Lord. The mind is most difficult to control and one has to conquer it otherwise lust, anger, avarice and illusion deviate him. He further explains that controlling ones mind is possible through the practice of yoga, with firm faith & determination.
In order to practice yoga one has to sit in a secluded place and meditate on the Paramatma or the Inner soul. One should hold one's body, neck and head erect and stare steadily at the tip of nose with an un-agitated and subdued mind. One should regulate the habits of eating, sleeping, recreation and work, and free oneself from material desires. Like a lamp in the windless place, one has to steadily meditate on the Paramatma with a controlled mind. If the mind wanders, one should bring it back under the control of the self. The natural boundless transcendental pleasure is the highest perfection or ultimate goal of Yoga.
Lord Krishna further states that a person who attains this transcendental knowledge sees everybody as a part and parcel of Krishna. A person who knows Krishna as supreme attains peace from the pangs of material miseries.