Catalyzes the conversion of (2E,6E)-farnesyl diphosphate (FPP) into the sesquiterpene alcohols (-)-alpha-gurjunene and 5-hydroxy-alpha-gurjunene (PubMed:27650333). Other unidentified sesquiterpene alcohols found to be catalyzed by MTPSL4 may arise from carbocation reaction intermediates along the catalytic cascade to gurjunene being quenched by a water molecule, yielding formation of the alcohols (Probable). (2E,6E)-farnesyl diphosphate = (-)-alpha-gurjunene + diphosphate (2E,6E)-farnesyl diphosphate + H2O = 5-hydroxy-alpha-gurjunene + diphosphate Binds 3 Mg(2+) ions per subunit. kcat is 0.058 sec (-1) with (2E,6E)-farnesyl diphosphate as substrate. Secondary metabolite biosynthesis; terpenoid biosynthesis. Belongs to the terpene synthase family.