Patanjali now details the 4th kind of pranayama,
Baahyaabhyantaravishayaakshepi: That which happens when one gives up on both internal and external subject
Chaturthah: The fourth
When one gives up on the awareness of the internal as well as external subjects, the pranayama that happens then is the fourth kind of pranayama.
When one loses awareness of the internal and external matters including the breath, and becomes still on one point /focus, at that time when the breath stops automatically without the knowledge of place, time or count, that is the fourth pranayama. In this state as the restlessness of mind comes to halt, the flow of prana ceases on its own, without conscious effort.