Neural stem cell (NSC) state and fate depend on spatially and temporally synchronized transcriptional and epigenetic regulation of the expression of extrinsic signaling factors and intrinsic cell-specific genes, but the functional roles for chromatin-modifying enzymes in neural differentiation remain poorly understood. Here we show that the histone demethylases KDM4A (JMJD2A) and KDM4C (JMJD2C) are essential for proper differentiation of NSCs in vitro and in vivo. KDM4A/C were required for neuronal differentiation, survival and expression of the neurotrophic signaling factor BDNF in association with promoter H3K9 demethylation and RNA polymerase II recruitment. Unexpectedly, KDM4A/C were essential for selective H3K36 demethylation and loss of RNA polymerase II recruitment in transcribed regions of the astrocyte-characteristic gene GFAP, thereby in parallel repressing astrocytic differentiation by control of elongation. We propose that gene- and lysine-specific KDM4A/C-mediated control of histone methylation and thereby regulation of intrinsic factors and signaling factors such as BDNF provide a novel control mechanism of lineage decision.