The S-3 state is currently the last observable intermediate prior to O-O bond formation at the oxygen-evolving complex (OEC) of Photosystem II, and its electronic structure has been assigned to a homovalent Mn-4(IV) core with an S = 3 ground state. While structural interpretations based on the EPR spectroscopic features of the S-3 state provide valuable mechanistic insight, corresponding synthetic and spectroscopic studies on tetranuclear complexes mirroring the Mn oxidation states of the S-3 state remain rare. Herein, we report the synthesis and characterization by XAS and multifrequency EPR spectroscopy of a (Mn4O4)-O-IV cuboidal complex as a spectroscopic model of the S-3 state. Results show that this (Mn4O4)-O-IV complex has an S = 3 ground state with isotropic 33Mn hyperfine coupling constants of -75, -88, -91, and 66 MHz. These parameters are consistent with an alpha alpha alpha beta spin topology approaching the trimer monomer magnetic coupling model of pseudo -octahedral Mn-IV centers. Importantly, the spin ground state changes from S = 1/2 to S = 3 as the OEC is oxidized from the S-2 state to the S-3 state. This same spin state change is observed following oxidation of the previously reported (MnMn3O4)-Mn-III-O-IV cuboidal complex to the (Mn4O4)-O-IV complex described here. This sets a synthetic precedent for the observed low-spin to high -spin conversion in the OEC.