The contribution of a 3d(4) spin configuration to the valence electronic structure of Fe compounds can be probed via spin-selective Fe K-pre-edge absorption spectra, using resonant inelastic X-ray scattering (RIXS). The 3d(4) configuration of Fe(IV) can be unequivocally detected even in a mixture with the high-spin 3d(5) configuration of Fe(III). This is demonstrated on the perovskite FeSrO3-x with formal oxidation state Fe(IV). When the technique was applied to an Fe-ZSM-5 catalyst during reaction with N2O, no 3d(4) configuration was detected. The formation of Fe(IV) upon reaction of Fe-ZSM-5 with N2O can, therefore, be ruled out.