We have previously developed a cocktail of nine small molecules to convert human fetal astrocytes into neurons, but a nine-molecule recipe is difficult for clinical applications. Here, we identify a chemical formula with only three to four small molecules for astrocyte-to-neuron conversion. We demonstrate that modulation of three to four signaling pathways among Notch, glycogen synthase kinase 3, transforming growth factor β, and bone morphogenetic protein pathways is sufficient to change an astrocyte into a neuron. The chemically converted human neurons can survive >7 months in culture, fire repetitive action potentials, and display robust synaptic burst activities. Interestingly, cortical astrocyte-converted neurons are mostly glutamatergic, while midbrain astrocyte-converted neurons can yield some GABAergic neurons in addition to glutamatergic neurons. When administered in vivo through intracranial or intraperitoneal injection, the four-drug combination can significantly increase adult hippocampal neurogenesis. Together, human fetal astrocytes can be chemically converted into functional neurons using three to four small molecules, bringing us one step forward for developing future drug therapy.