S-adenosyl-L-methionine-dependent protein-arginine N-methyltransferase that methylates the delta-nitrogen atom of arginine residues to form N5-methylarginine (type IV) in target proteins. Monomethylates ribosomal protein L12. Monomer. Belongs to the class I-like SAM-binding methyltransferase superfamily. RMT2 methyltransferase family.