The fourth [type of prāṇāyāma] transcends the domain of external and internal.

After describing the three types of prāṇāyāma (exhalation, inhalation, retention), Patañjali mentions a more subtle fourth.

Caturtha is “the fourth.” This prāṇāyāma transcends (ākṣepī, literally “throws beyond”) the domains (viṣaya) of external and internal.

It’s not a technique practiced voluntarily; it arises spontaneously when practice matures. Breathing suspends naturally, without effort or decision.

This kevala kumbhaka (spontaneous retention) is a state where the distinction between inside and outside dissolves. Prāṇa becomes completely still.

It is the threshold toward higher states of consciousness, where breath practically ceases.