Fourfold spheres of perception. Same as the four formless realms. The four unenlightened meditative states of dwelling on the thoughts: Infinite Space, Infinite Consciousness, Nothing Whatsoever, and Neither Presence Nor Absence of conception
{gzugs med pa'i khams bzhi}: {nam mkha' mtha' yas, rnam shes mtha' yas, ci yang med pa, yod min med min te skye mched mu bzhi'o} The four formless realms are the four perception-spheres of Infinite Space, Infinite Consciousness, Nothing Whatsoever, and Neither Presence Nor Absence [of Perception]
the four perception spheres. See {gzugs med pa'i khams bzhi, gzugs med pa'i snyoms par 'jug pa bzhi}