Adaptor protein 4 (AP-4) is a heterotetrameric complex composed of ε, β4, μ4, and σ4 subunits that mediates export of a subset of transmembrane cargos, including autophagy protein 9A (ATG9A), from the trans-Golgi network (TGN). AP-4 has received particular attention in recent years because mutations in any of its subunits cause a complicated form of hereditary spastic paraplegia referred to as "AP-4-deficiency syndrome." The identification of proteins that interact with AP-4 has shed light on the mechanisms of AP-4-dependent cargo sorting and distribution within the cell. However, the mechanisms by which the AP-4 complex itself is assembled have remained unknown. Here, we report that the alpha- and gamma-adaptin-binding protein (AAGAB, also known as p34) binds to and stabilizes the AP-4 ε and σ4 subunits, thus promoting complex assembly. The physiological importance of these interactions is underscored by the observation that AAGAB-knockout cells exhibit reduced levels of AP-4 subunits and accumulation of ATG9A at the TGN like those in cells with mutations in AP-4-subunit genes. These findings demonstrate that AP-4 assembly is not spontaneous but AAGAB-assisted, further contributing to the understanding of an adaptor protein complex that is critically involved in development of the central nervous system.