Catalyzes the transfer of a farnesyl moiety from farnesyl diphosphate to a cysteine at the fourth position from the C-terminus of several proteins. The beta subunit FTB is responsible for peptide-binding. (2E,6E)-farnesyl diphosphate + L-cysteinyl-[protein] = diphosphate + S-(2E,6E)-farnesyl-L-cysteinyl-[protein] Binds 1 zinc ion per subunit. Heterodimer of FTA and FTB. Belongs to the protein prenyltransferase subunit beta family.