Not to be confused with the Casio Micro-Mini Calculator that is used by Egon in the scene Fixer-Upper.
After their encounter with the Library ghost at the New York City Public Library, the Ghostbusters returned to Columbia University. Egon used the PC-4 to make some calculations with data collected on the P.K.E. Meter. The computations confirmed that a ghost could be captured and held in storage indefinitely, assuming the ionization rate remained constant.