Biphenyl compound as immunomodulator, preparation method therefor and application thereof

A compound of formula (I)

• or its stereoisomers, pharmaceutically acceptable salts and metabolites,
• wherein
• R1 and R2 can be identically or differently selected from C1-C6 alkyl, cyano, and halogen;
• R3 is selected from hydrogen, C1-C6 alkyl, C2-C6 alkenyl C1-C6 alkyl, C2-C6 alkynyl C1-C6 alkyl, C1-C6 haloalkyl, C1-C6 alkoxy C1-C6 alkyl, mono-C1-C6 alkylamino C1-C6 alkyl, di-C1-C6 alkylamino C1-C6 alkyl, C1-C6 haloalkoxy C1-C6 alkyl, C3-C14 cycloalkyl, 3- to 14-membered heterocycloalkyl, C3-C14 cycloalkyl-C1-C4 alkyl, and 3- to 14-membered heterocycloalkyl-C1-C4 alkyl;
• R4 is selected from hydrogen and C1-C6 alkyl, wherein the C1-C6 alkyl is optionally substituted by one or more substituents selected from hydroxyl group, carboxy and halogen;
• X is selected from —O—, —S—, and —N(Ra)—;
• Ra is selected from hydrogen, C1-C6 alkyl, and C1-C6 haloalkyl;
• m is 1, 2 or 3;
• n is 1, 2 or 3.