HTR4 (5-hydroxytryptamine receptor 4, 5-HT4) is a G protein-coupled receptor (GPCR) belonging to the serotonin receptor family. Activated by the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT), HTR4 primarily couples to Gs proteins, leading to stimulation of adenylyl cyclase, increased intracellular cAMP levels, and activation of downstream signaling pathways. HTR4 is widely expressed in the gastrointestinal tract, central nervous system, and cardiovascular system, where it regulates gastrointestinal motility, neurotransmission, learning and memory, and cardiac function. In the nervous system, HTR4 has been implicated in cognitive processes and neuroprotection, while in the gut it promotes intestinal peristalsis and secretion.