The AN/PRR-4 was an aperiodic radio receiver, developed in the US
in the early 1950s and used from 1952 onwards as a countermeasures receiver
for finding bugs in the GHz range, such as the
Great Seal Bug
(a.k.a. The Thing),
planted by the Soviets in the office of the US Ambassador in Moscow
on 4 August 1945 and discovered by the US State Department
on 12 September 1952.
Currently, no further information about the PRR-4 is available to us.