Component of the adaptor protein complex 4 (AP-4). Adaptor protein complexes are vesicle coat components involved both in vesicle formation and cargo selection. They control the vesicular transport of proteins in different trafficking pathways. AP-4 forms a non clathrin-associated coat on vesicles departing the trans-Golgi network (TGN) and may be involved in the targeting of proteins from the trans-Golgi network (TGN) to the endosomal-lysosomal system (By similarity). It is also involved in protein sorting to the basolateral membrane in epithelial cells and the proper asymmetric localization of somatodendritic proteins in neurons (PubMed:18341993). Within AP-4, the mu-type subunit AP4M1 is directly involved in the recognition and binding of tyrosine-based sorting signals found in the cytoplasmic part of cargos. The adaptor protein complex 4 (AP-4) may also recognize other types of sorting signal (By similarity). Adaptor protein complex 4 (AP-4) is a heterotetramer composed of two large adaptins (epsilon-type subunit AP4E1 and beta-type subunit AP4B1), a medium adaptin (mu-type subunit AP4M1) and a small adaptin (sigma-type AP4S1). Interacts with tyrosine-based sorting signals on the cytoplasmic tail of cargo proteins such as APP, ATG9A, LAMP2 and NAGPA. Interacts with the C-terminal domain of GRID2 (By similarity). Interacts with GRIA1 and GRIA2; the interaction is indirect via CACNG3 (PubMed:18341993). Interacts with CACNG3; CACNG3 associates GRIA1 and GRIA2 with the adaptor protein complex 4 (AP-4) to target them to the somatodendritic compartment of neurons (PubMed:18341993). Interacts with HOOK1 and HOOK2; the interactions are direct, mediate the interaction between FTS-Hook-FHIP (FHF) complex and AP-4 and the perinuclear distribution of AP-4 (By similarity). Found in soma and dendritic shafts of neuronal cells. Belongs to the adaptor complexes medium subunit family.