Membrane-anchoring mono-heme cytochrome b subunit of succinate dehydrogenase (SDH) that is involved in system II of the mitochondrial electron transport chain and is responsible for transferring electrons from succinate to ubiquinone (coenzyme Q). SDH3 and SDH4 form the membrane dimer that anchors the catalytic dimer formed by SDH1 and SDH2 to the matrix surface of the mitochondrial inner membrane. Electrons originating from the catalytic dimer enter the membrane dimer for ubiquinone reduction. The heme is bound between the two transmembrane subunits. Carbohydrate metabolism; tricarboxylic acid cycle. Forms part of complex II containing four subunits: a flavoprotein (FP), an iron-sulfur protein (IP) and a cytochrome b composed of two integral membrane proteins. Present with 238 molecules/cell in log phase SD medium. Belongs to the cytochrome b560 family.