Sulfur K-edge X-ray absorption spectroscopy (XAS) is reported for [Fe4S4](1+,2+,3+) clusters. The results are quantitatively and qualitatively compared with DFT calculations. The change in covalency upon redox in both the [Fe4S4](1+/2+) (ferredoxin) and the [Fe4S4](2+/3+) (HPIP) couple are much larger than that expected from just the change in number of 3d holes. Moreover, the change in the HiPIP couple is higher than that of the ferredoxin couple. These changes in electronic structure are analyzed using DFT calculations in terms of contributions from the nature of the redox active molecular orbital (RAMO) and electronic relaxation. The results indicate that the RAMO of HiPIP has 50% ligand character, and hence, the HiPIP redox couple involves limited electronic relaxation. Alternatively, the RAMO of the ferredoxin couple is metal-based, and the ferredoxin redox couple involves extensive electronic relaxation. The contributions of these RAMO differences to ET processes in the different proteins are discussed.