Catalyzes the transfer of a geranyl-geranyl moiety from geranyl-geranyl pyrophosphate to a cysteine at the fourth position from the C-terminus of proteins having the C-terminal sequence Cys- aliphatic-aliphatic-X. Known substrates include RAC1, RAC2, RAP1A and RAP1B. . This is the function of PGGT1B (protein geranylgeranyltransferase type I subunit beta, ENSG00000164219).