Specific class of high-redox-potential 4Fe-4S ferredoxins. Functions in anaerobic electron transport in most purple and in some other photosynthetic bacteria and in at least one genus (Paracoccus) of halophilic, denitrifying bacteria. Competent in photosynthetic electron transfer to oxidized cytochrome bc1 complex via the membrane-bound c-type tetraheme. E(0) is 351 +/-16 mV. Homodimer (By similarity). Monomer at different ionic strengths (PubMed:7574702). Belongs to the high-potential iron-sulfur protein (HiPIP) family.