Catalysis of the reaction: L-lysyl20- + S-adenosyl-L-methionine = H+ + N6-methyl-L-lysyl20- + S-adenosyl-L-homocysteine. This reaction is the addition of a methyl group to the unmethylated lysine residue at position 20 of histone H4, producing histone H4K20me.
histone H4K20 monomethylation