Multiheme proteins are common in bacteria, especially those that function in electron-rich environments. Cytochrome (cyt) cbb3 oxidases catalyze O2 reduction in microoxic environments and, in Pseudomonas and other bacteria, these enzymes have a chain of five hemes and its electron donor cyt c4 has two. We have employed a combination of spectroscopic, electrochemical, and computational methods as well as in vivo studies to characterize redox properties of individual hemes in this system and define the role of interdomain interface and conformational dynamics in directing electron flow. We have mapped interactions formed in the cbb3-c4 ET complex and demonstrated how electron injection to the enzyme benefits from the diheme architecture of c4. Connecting c4-A and c4-B fragments in the full-length protein upshifts the potentials of the heme iron, assists in folding of the domains, and prevents their homodimerization. Critical in establishing the interdomain interface in c4 are hydrogen bonds between heme propionates (HPs) of c4-A and c4-B. While our photoinduced ET studies of Ru-labeled c4 revealed that interheme ET rates are affected by pH, the NMR studies suggested that changes in 13C chemical shifts are smaller than those expected for deprotonation of HPs. Further, the transitions were the same for both ferric and ferrous proteins, arguing against the role of HPs in proton-coupled ET. The pH effects on ET instead stem from changes in protein dynamics. Molecular dynamics simulations identified structural elements at the interdomain interface connected to the heme iron ligand that may respond to changes in the redox state of the neighboring domain. Studies of c4 variants probed the role of redox cooperativity and determined that it could be enhanced through changes at the protein surface, implying a mechanism of ET control through interactions in redox complexes.