The Salyut-4 X-ray instrument, often called the Filin telescope, consisted of four gas flow proportional counters, three of which have a total detection surface of 450 sq cm in the energy range 2-10 keV, and one of which has an effective surface of 37 sq cm. The field of view was limited by a slit collimator to 3 deg x 10 deg FWHM. The instrumentation also included optical sensors which were mounted on the outside of the station together with the X-ray detectors, and power supply and measurement units which were inside the station. Ground based calibration of the detectors is considered along with in-flight operation in three modes: inertial orientation, orbital orientation, and survey.