Bahyabhyantara-visayaksepi caturthah (II/51)
Bahya = external; abhyantara= internal; visaya = sphere; aksepi = going beyond; caturthah = the fourth variety of kumbhaka.
That pranayama (kumbhaka) which goes beyond the sphere of internal and external is the fourth.
We move from gross to subtle levels as we progress on the path of yoga. There are various shades of experience as we go deep in pranayama. Initially kumbhaka (holding of the breath) is performed with effort, after inhalation (antah kumbhaka) or exhalation (bahya kumbhaka). When holding of the breath becomes natural (sahaj), we call it kevala kumbhaka. Kevala kumbhaka can take place at any point between inhalation and exhalation. Many a time we experience a glimpse of suspension of breath during moments of bliss when thoughts cease to exist e.g. while seeing a beautiful scene (that is why we exclaim, "the scene was breath taking"), listening to the climax of an interesting episode or classical music and so on.
As we go on taming the forces of prana, the kumbhaka gets more and more effortless and refined. We may begin by breathing out, breathing in and by holding the breath but eventually, we learn to perform the refined type of kumbhaka (chaturthah) without involving any breathing at gross level. This goes along with our perception and then manipulation of subtle pranic forces. In fact real practice of pranayama begins when we learn to handle this subtle prana. This happens after we acquire mastery over prana. Then we can perform this caturthah type, effortless kumbhaka of prana anywhere in the system like the masters who can hold their mind (dharana) at any point.
One adept in prana-vidya, when suffering from headache, used to refer to his ailment in this fashion: "today too much of prana is rushing to the upper region; let me withdraw it from there".