Subunit of novel type of clathrin- or non-clathrin-associated protein coat involved in targeting proteins from the trans-Golgi network (TGN) to the endosomal-lysosomal system. Adaptor protein complex 4 (AP-4) is a heterotetramer composed of two large adaptins (epsilon-type subunit and beta-type subunit), a medium adaptin (mu-type subunit) and a small adaptin (sigma-type subunit) (By similarity). Interacts with EHD2 (PubMed:19936242). Associated with the trans-Golgi network. Belongs to the adaptor complexes small subunit family.