Catalyzes the methylation of C-11 in cobalt-precorrin-4 to form cobalt-precorrin-5A. Co-precorrin-4 + S-adenosyl-L-methionine = Co-precorrin-5A + H(+) + S-adenosyl-L-homocysteine Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 4/10. Homodimer. Belongs to the precorrin methyltransferase family.