Lysine-specific demethylase 4B is an enzyme that in humans is encoded by the KDM4B gene. KDM4B belongs to the alpha-ketoglutarate-dependent hydroxylase superfamily. Histone demethylase that specifically demethylates 'Lys-9' of histone H3, thereby playing a role in histone code. Does not demethylate histone H3 'Lys-4', H3 'Lys-27', H3 'Lys-36' nor H4 'Lys-20'. Only able to demethylate trimethylated H3 'Lys-9', with a weaker activity than KDM4A, KDM4C and KDM4D. Demethylation of Lys residue generates formaldehyde and succinate. Plays a critical role in the development of the central nervous system (CNS).