The fourth is restraining the Prana by reflecting on external or internal object.
In plain words: There is a fourth kind of breath control. Here the breath settles and stops on its own after long practice, while the mind reflects on something outside or inside the body.
This is the fourth sort of Pranayama, in which the Kumbhaka is brought about by long practice attended with reflection, which is absent in the other three.
Namaste.
A calm guide to life, the mind and the old teachings.