The scheme of arrangement of a four-circle diffractometer on a thermal neutron beam from the HEC-9 channel of the PIK reactor is proposed. Calculations are performed using analytical and numerical approaches. The setup parameters are optimized: the resolution, the size of the focusing monochromator and its curvature in the vertical and horizontal directions, the efficiency of filters, and the monochromatic neutron-flux density on the sample, with which the maximum intensity is achieved in the detector. It is shown that the beam intensity at the position of the detector is a more important parameter than the intensity at the sample position during device optimization. The device characteristic obtained will allow studying the atomic and magnetic structure of a wide class of crystals.