Methyltransferase that likely catalyzes the ring contraction and methylation of C-17 in cobalt-factor III to form cobalt-factor IV. May also convert cobalt-precorrin-3 to cobalt-precorrin-4. Co(II)-factor III + H(+) + S-adenosyl-L-methionine = Co(II)-factor IV + S-adenosyl-L-homocysteine Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 3/10. Belongs to the precorrin methyltransferase family.