Core component of the TRAPP complexes which has a function of guanine nucleotide exchange factor activity for Rab1 GTPase. Plays a role in vesicular transport from endoplasmic reticulum to Golgi and autophagy (By similarity). May play a role in dendrite postsynaptic membrane trafficking (PubMed:11018053). Component of the multisubunit TRAPP (transport protein particle) complex, which includes at least TRAPPC2, TRAPPC2L, TRAPPC3, TRAPPC3L, TRAPPC4, TRAPPC5, TRAPPC8, TRAPPC9, TRAPPC10, TRAPPC11 and TRAPPC12 (By similarity). Interacts with SDC2 (PubMed:11018053). Associated with postsynaptic membranes and in intracellular cisterns and vesicles (Golgi). Widely expressed. Belongs to the TRAPP small subunits family. TRAPPC4 subfamily.