Catalysis of the reaction: histone H3 N6-methyl-L-lysine (position 4) + O2 + FAD + H2O = histone H3 L-lysine (position 4) + H2O2 + formaldehyde + FADH2. This reaction is the removal of a methyl group from monomethylated lysine at position 4 of the histone H3 protein.