What is the main purpose of a saint's incarnation, and what happens when their work is complete, based on Chapter 4?

The main purpose of a saint's incarnation, as detailed in Chapter 4 of the Shri Sai Satcharitra, is to protect the system of castes and stages of life, destroy irreligion, and protect the humble, poor, and weak. Saints are described as naturally liberated beings who descend to the mortal world purely for the sake of others, with no self-interest involved. Their mission is to awaken religion and faith when humanity has lost its way. The chapter concludes this point by stating that once their specific work is fulfilled, they complete their mission and end their incarnation, having restored the foundation for spiritual upliftment.