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 (PubMed:10066790, PubMed:10436028, PubMed:11139587, PubMed:11802162, PubMed:20230749). 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 (PubMed:11139587, PubMed:20230749). It is also involved in protein sorting to the basolateral membrane in epithelial cells and the proper asymmetric localization of somatodendritic proteins in neurons (By similarity). 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 (PubMed:10436028, PubMed:11139587, PubMed:20230749, PubMed:26544806). The adaptor protein complex 4 (AP-4) may also recognize other types of sorting signal (By similarity). {ECO:0000250|UniProtKB:E2RED8, ECO:0000250|UniProtKB:Q2PWT8, ECO:0000250|UniProtKB:Q9JKC7, ECO:0000269|PubMed:10066790, ECO:0000269|PubMed:10436028, ECO:0000269|PubMed:11139587, ECO:0000269|PubMed:11802162, ECO:0000269|PubMed:20230749, ECO:0000269|PubMed:26544806}. This is the function of ENSG00000221838 (AP4M1, adaptor related protein complex 4 subunit mu 1).