N4-methylcytidine (m4C) methyltransferase responsible for the methylation of position C839 in mitochondrial 12S rRNA. Involved in the stabilization of 12S rRNA folding, therefore facilitating the assembly of the mitochondrial small ribosomal subunits. cytidine(839) in 12S rRNA + S-adenosyl-L-methionine = H(+) + N(4)-methylcytidine(839) in 12S rRNA + S-adenosyl-L-homocysteine Belongs to the methyltransferase superfamily. RsmH family.