Study a single sutra by selecting the pada (chapter) and then the sutra (verse).
Source: English translation from Light on the Yoga Sutras of Patanjali (B.K.S. Iyengar)
The fourth type of pranayama transcends the external and internal pranayamas, and appears effortless and non-deliberate.
Source: How to Know God: The Yoga Aphorisms of Patanjali (Swami Prabhavananda, Christopher Isherwood)
The fourth kind of pranayama is the stoppage of the breath which is caused by concentration upon external or internal objects.
Source: English translation of the Yoga Sutras of Patanjali (Swami Jnaneshvara Bharati)
The fourth pranayama is that continuous prana which surpasses, is beyond, or behind those others that operate in the exterior and interior realms or fields.