We theoretically discuss the capability of resonant photoemission with soft X-rays in clarifying the valence electronic state, by choosing Ce 4d core X-ray photoemission spectroscopy (4d XPS) at the Ce 3d threshold in CeRh3B2 with hexagonal crystal structure. On the basis of the Anderson model including multiplet splitting, we show that the dependence of 4d XPS on the energy and the linear polarization with respect to the hexagonal c-axis of the incident photon and the binding energy can be a powerful characteristic for probing a proposed uniaxial anisotropic distribution of 4f electrons.