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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., FRα, FRβ）, 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.