S1PR4 (sphingosine-1-phosphate receptor 4) is a G protein-coupled receptor that plays crucial roles in immune cell regulation and inflammatory responses. As a receptor for sphingosine-1-phosphate (S1P), S1PR4 modulates immune cell trafficking and activation across multiple cell types 1. The receptor is primarily expressed in immune cells, particularly CX3CR1+ CD8+ effector T cells, where S1P/S1PR4 signaling promotes cell migration and facilitates differentiation into tissue-resident memory T cells 2. In myeloid cells, S1PR4 attenuates pro-inflammatory cytokine production in macrophages and regulates neutrophil activation, demonstrating anti-inflammatory effects 1. S1PR4 also influences neutrophil trafficking and circulating counts, as evidenced by genetic variants affecting neutrophil levels 3. Therapeutically, S1PR4 represents a promising target for inflammatory diseases. Etrasimod, an S1PR1/4/5 modulator, has shown efficacy in ulcerative colitis by inhibiting lymphocyte trafficking from lymph nodes 4. In biliary atresia models, S1PR4 inhibition reduced liver inflammation and fibrosis 2. The receptor's expression has also been identified as a potential biomarker for coronary artery disease 5, highlighting its broader clinical relevance beyond immune disorders.