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The adaptor protein 4 (AP4) complex (/[beta]4/µ4/σ4 subunits) forms a non-clathrin coat on vesicles departing the trans-Golgi network. AP4 biology remains poorly understood, in stark contrast to the wealth of molecular data available for the related clathrin adaptors AP1 and AP2. AP4 is important for human health because mutations in any AP4 subunit cause severe neurological problems, including intellectual disability and progressive spastic para- or tetraplegias. We have used a range of structural, biochemical and biophysical approaches to determine the molecular basis for how the AP4 [beta]4 C-terminal appendage domain interacts with tepsin, the only known AP4 accessory protein. We show that tepsin harbors a hydrophobic sequence, LFxG[M/L]x[L/V], in its unstructured C-terminus, which binds directly and specifically to the C-terminal [beta]4 appendage domain. Using nuclear magnetic resonance chemical shift mapping, we define the binding site on the [beta]4 appendage by identifying residues on the surface whose signals are perturbed upon titration with tepsin. Point mutations in either the tepsin LFxG[M/L]x[L/V] sequence or in its cognate binding site on [beta]4 abolish in vitro binding. In cells, the same point mutations greatly reduce the amount of tepsin that interacts with AP4. However, they do not abolish the binding between tepsin and AP4 completely, suggesting the existence of additional interaction sites between AP4 and tepsin. These data provide one of the first detailed mechanistic glimpses at AP4 coat assembly and should provide an entry point for probing the role of AP4-coated vesicles in cell biology, and especially in neuronal function.