Yoga is meditation where mind focus on object of meditation till the culmination to Asamprajnata Samadhi (no devision between the meditator and object of meditation). In that state, the meditator stays in his/her true nature, otherwise he/she identifies with the mind. This is the summary from sutra no 1-4.
To understand sutra 3 and 4 more clearly, we need to understand what is the nature of Ātma which is me, and what is anātma which is not me. Let us see this through the method of creation.
Brahman which is pure consciousness has the power to create which called Maya or Prakrti. Maya is endowed with 3 Guna, which are satva, rajas and tamas. Through this power of creation, the entire universe including our gross physical body and subtle body (vital energy, senses and mind) are the manifested with 3 guna. There is no separate entity between the intelligent cause and the material cause of the universe, unlike the pot maker is the intelligent cause (as the knowledge of the pot must be in him), and the clay is the material cause for the pot. Pot maker and the clay are separate entity. Brahman itself is the intelligent and material cause of the universe, and maya is the power of projection. Just like our my dream. There is no other separate material for us to create the dream world except my knowledge alone and my power of projection. Therefore my dream is having its nature as me alone the conscious being, and in the same manner the entire universe including me and you, is having our nature as consciousness Brahman alone.
Therefore any attribute like body, prana, and mind are not me. They are superimposed upon me the pure consciousness. But why this superimposition can happened? The key factor here is ahamkara / ego / I-notion which also the result of maya. Because of the I-notion, the Self identifies with non-self which has proximity to the Self. Just like a clear crystal appears to be red when there is a red hibiscus flower in proximity.
Therefore the sutra no 4 said that one is identify with the mind, when one is not staying in his/her true nature. But there is a different between Yoga and vedanta in this approach. Yoga is stopping the mind to avoid identification, but vedanta teaching is to see the fact and cognitively separates between ātma and anātma. We don't need to take away the red hibiscus flower from the crystal in order to know the nature of crystal is clear.