Folate receptor 4 (FR4), also known as FOLR4, is a glycosylphosphatidylinositol (GPI)-anchored cell surface protein belonging to the folate receptor family. It is primarily expressed in the immune system, particularly in regulatory T cells (Tregs) and certain B cell subsets. The function of FR4 has not been fully elucidated, but studies suggest it may play a role in immune regulation and cell signaling. Unlike other members of the family (e.g., FR伪, FR尾), FR4 has a lower affinity for folate, indicating it may have unique biological functions. Additionally, FR4 expression is associated with certain cancers and autoimmune diseases, potentially serving as a diagnostic marker or therapeutic target. Research is ongoing to explore its specific mechanisms and clinical applications.