This study presents the results of a complex physical modeling of a moderate power gyrotron operating at the 4-mm wavelength range. The characteristics of electrodes and magnetic coils in a four-stage recovery collector were optimized taking into account the coordinate and velocity distributions of electrons. These distributions were obtained through a trajectory analysis in the electron optical system and calculation of electron-wave interaction in the gyrotron cavity. To reduce parasitic effects of the bundles of a toroidal solenoid used to create an azimuthal magnetic field in the collector region, a sectioned electron beam was employed. The study demonstrated that the gyrotron's total efficiency of approximately 79 % could be achieved, being close to the maximum efficiency value achievable with separation of electron fractions with different energies, provided that the current of electrons reflected from a collector should not exceed 1% of the total current of an electron beam.