GO:0035575

Catalysis of the reaction: histone H4 N6-methyl-L-lysine (position 20) + 2-oxoglutarate + O2 = histone H4 L-lysine (position 20) + succinate + formaldehyde + CO2. This reaction is the removal of a methyl group from lysine at position 20 of the histone H4 protein. [EC:1.14.11.27, PMID:20622853]