Letters on Yoga—IV contains letters of Sri Aurobindo on the transformation of human nature – mental, vital, and physical – through the practice of the Integral Yoga, and on the difficulties encountered along the way. It is the last volume of Letters on Yoga. Most of these letters were written by Sri Aurobindo in the 1930s to disciples living in his Ashram in Pondicherry. About one-third of the letters in the present volume were not published in the Sri Aurobindo Birth Centenary Library.

This fourth volume is arranged by subject in four parts. The first covers sadhana on the level of the mind (mental difficulties, cultivation of the mind, power of expression, and speech), and is followed by sadhana on the level of the vital (the vital nature, vital temperament, and vital defects; human relations and interactions). The third part deals with sadhana on the physical, subconscient, and inconscient levels and the fourth addresses the kinds of difficulties encountered in the practice of Yoga, how to face and overcome such difficulties, and the nature and function of the adverse forces in the sadhana.