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. May interact with PATJ, MAGI2, MPP3, NOS1, SLC9A3R1, SEC23A and SNX27. May form a complex including SLC9A3R1 and EZR (By similarity). 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. Additional isoforms seem to exist. In brain, isoform 5-HT4S is restricted to the striatum, but isoform 5-HT4L is expressed throughout the brain, except in the cerebellum. In peripheral tissues, differential expression is also observed in the atrium of the heart where only isoform 5-HT4S is detectable. Belongs to the G-protein coupled receptor 1 family.