This is the first complete English translation of the fourth chapter of the esoteric Buddhist Kalacakra Tantra text and its eleventh-century commentary, the Stainless Light (Vimalaprabha). Building upon the Chapter on the Cosmos and particularly the Chapter on the Individual (AIBS, 2004)--which provide the theoretical background to the Chapter on Sadhana, and the reasons for the given structure and contents of the Kalacakra sadhana practice--this fourth chapter illuminates the intricate connection between the practice of the Kalacakra sadhana and the Kalacakra Tantra's worldview. This fourth chapter describes Buddhist Tantric generation stage practices (utpatti-krama), including instructions on protecting the place of practice, the meditative practices of the origination of the body and the deities abiding in the body, and the diverse mundane sadhanas designed to induce the mundane siddhis. It then also describes the more advanced Buddhist Tantric completion stage practices (sampatti-krama), designed to lead directly to the attainment of buddhahood, called here the "Adibuddha" (Primordial Buddha). The translation is supplemented with annotations and references to Tibetan commentaries and other esoteric Buddhist works. It also includes the first critical edition of the Mongolian version of the fourth chapter. Published by American Institute of Buddhist Studies (AIBS)