Histone demethylase that specifically demethylates dimethylated 'Lys-20' of histone H4 (H4K20me2), thereby modulating chromosome architecture. Binds 1 Fe(2+) ion per subunit. Phosphorylated by PKA. Belongs to the round spermatid basic protein 1 family. Truncated N-terminus. Aberrant splicing.