The function of S1PR4 (sphingosine-1-phosphate receptor 4, ENSG00000125910) is as follows. G protein-coupled receptor highly expressed in immune cells, where it regulates immune response and cytokine production. Functions as a receptor for the lysosphingolipid sphingosine-1-phosphate (S1P). Upon S1P binding, promotes regulatory T-cell differentiation and enhances fatty acid oxidation, through activation of the NRF2/PPARA signaling pathway (By similarity). Modulates also M1 macrophage activation through interaction with FPR2 and the JNK signaling, contributing to the inflammatory response (By similarity). In addition, facilitates early neutrophil mobilization and vascular activation during inflammation, promoting lymphocyte recruitment to draining lymph nodes and supporting the development of germinal centers for an effective adaptive immune response (By similarity). {ECO:0000250|UniProtKB:Q9Z0L1, ECO:0000269|PubMed:10679247, ECO:0000269|PubMed:10753843}.