Let us first understand the Jargon ” Bhakti Yoga, Karma yoga,Dhyana and Gyana yoga” of Spiritual science in a simplified way.
An expert driver is perfect in driving. When you have to learn perfect driving then you go to the expert driver, trust him, listen to him and follow him and gradually through learning and practicing you become an expert.
What do you do here ?
You start from where you are with the goal where you want to reach(in this case being an expert) and by following the one who is at the goal ,you eventually reach the goal.
This is true in all cases of goals in life whether it is material goals or the spiritual goal.
In the spiritual case, the goal is Self realisation or God(completeness), the path is Dharma, and the trust is devotion.
Just like in the case of learning driving, you find an expert driver, trust him, learn from him, follow him and gradually through learning and practicing you become an expert. Similarly in the case of learning how to remain perpetually blissful and peaceful from within, You find the Guru(Expert or Master), trust him, listen to him, follow him and gradually through learning and practicing you become an expert or master.
The Jargon used in the spiritual Science is “Bhakti Yoga, Karma yoga,Dhyana yoga and Gyana yoga”. Guru is an ideal who is at the goal, trusting him is devotion which is called Bhakti Yoga, following him is Dharma which is called Karma yoga , listening to him is called Gyana yoga and meditating on the teachings is Dhyana Yoga .
By Practicing Bhakti Yoga, Karma yoga , Gyana Yoga and Dhyana Yoga, you can attain the state of Guru or Self realisation(God) or Completeness. Bhakti(devotion) Yoga is the basis of the other three Yoga which are Karma(along Dharma) , Dhyana and Gyana Yoga. One who is on the path of Dharma follows Nishkama Karma which is Karma without attachment to the material fruits or results. And the one who has attained Self realisation or the goal, his action automatically becomes Nishkama Karma or Dharma.
Action or Karma performed from the state of Self realisation is free from expectations out of the completeness(God) within. Such Karma is also called as “Leela” which is not done with Selfish intension but with Selfless intension . Such Karma is done by a Self realised being for the upliftment of human consciousness by showing the path of Dharma.
PS: Ideal is the one who is at the goal where you want to reach, devotion (trust) connects you to the ideal, dharma is following the path shown by the ideal, Dhyana is meditating on the teachings of the Ideal and Gyana is the knowledge of the ideal taught by the Ideal.
Thanks & Best Wishes.