Involved in exocytosis. Maybe by regulating the binding and fusion of secretory vesicles with the cell surface. The GTP-bound form of SEC4 may interact with an effector, thereby stimulating its activity and leading to exocytotic fusion. SEC4 may be an upstream activator of the 19.5S SEC8/SEC15 particle. SEC4 probably interacts directly with SEC8; it could serve as the attachment site for the SEC8/SEC15 particle. Interacts with the guanyl-nucleotide exchange factor SEC2. Interacts with SRO7, YIF1, YIP3, YIP4 and YIP5. A small fraction is soluble. Belongs to the small GTPase superfamily. Rab family.