Histone demethylase that specifically demethylates 'Lys-4' of histone H3, thereby playing a central role in histone code (PubMed:17320161). Does not demethylate histone H3 'Lys-9', H3 'Lys-27', H3 'Lys-36', H3 'Lys-79' or H4 'Lys-20' (PubMed:17320161). Demethylates trimethylated and dimethylated but not monomethylated H3 'Lys-4' (PubMed:17320161). Required for normal longevity of the soma in a germline-dependent manner (PubMed:20555324, PubMed:22212395). Implicated in the epigenetic inheritance of lifespan over several generations (PubMed:22012258). Involved in larval development and vulva formation (PubMed:17320161). 3 2-oxoglutarate + N(6),N(6),N(6)-trimethyl-L-lysyl(4)-[histone H3] + 3 O2 = 3 CO2 + 3 formaldehyde + L-lysyl(4)-[histone H3] + 3 succinate Binds 1 Fe(2+) ion per subunit. RNAi-mediated knockdown decreases lifespan in wild type worms with an intact germline (PubMed:20555324). Results in extended lifespan when egg production is inhibited by the addition of floxuridine (FUDR) or in a glp-1 mutant background which lacks a germline (PubMed:22212395, PubMed:20555324). Belongs to the JARID1 histone demethylase family.