A protein complex consisting of four polypeptides which also contains tungsten, a molybdopterin guanine dinucleotide, and iron-sulfur clusters. This protein complex catalyzes the reversible conversion of CO2 and methanofuran to formylmethanofuran during methanogenesis. [ http://www.ncbi.nlm.nih.gov/pubmed/8125106 GOC:mengo_curators http://www.ncbi.nlm.nih.gov/pubmed/8575452 ]