This is one of the several different receptors for 5-hydroxytryptamine (serotonin), a biogenic hormone that functions as a neurotransmitter, a hormone, and a mitogen. The activity of this receptor is mediated by G proteins that stimulate adenylate cyclase (By similarity). Isoform 5-HT4(A) interacts with MAGI2, MPP3, SLC9A3R1 and SNX27 isoforms 1 and 2. Isoform 5-HT4(E) interacts with PATJ, NOS1 and SEC23A. Isoform 5-HT4(A) forms a complex including SLC9A3R1 and EZR. Interacts (via C-terminus 330-346 AA) with GRK5; this interaction is promoted by 5-HT (serotonin) (By similarity). Interaction with SNX27 mediates recruitment to early endosomes, while interaction with SLC9A3R1 and EZR might target the protein to specialized subcellular regions, such as microvilli. Belongs to the G-protein coupled receptor 1 family.