N4-methylcytidine (m4C) methyltransferase responsible for the methylation of position C839 in mitochondrial 12S rRNA (PubMed:31665743, PubMed:32371392). Involved in the stabilization of 12S rRNA folding, therefore facilitating the assembly of the mitochondrial small ribosomal subunits (PubMed:31665743, PubMed:32371392). {ECO:0000269|PubMed:31665743, ECO:0000269|PubMed:32371392}. This is the function of METTL15 (methyltransferase 15, mitochondrial 12S rRNA N4-cytidine, ENSG00000169519).