GO:0044020

Catalysis of the reaction: S-adenosyl-L-methionine + (histone H4)-arginine (position 3) = S-adenosyl-L-homocysteine + (histone H4)-N-methyl-arginine (position 3). This reaction is the addition of a methyl group to arginine at position 3 of histone H4. [EC:2.1.1.125, GOC:mah, PMID:17898714]