A compound of the formula (I) or a pharmaceutically acceptable salt thereof: wherein: R1 is hydrogen； a linear or branched C1-C7 alkyl group, optionally substituted with one or more substituents selected from the group consisting of hydroxy, halogen, cyano, C3-C7 cycloalkyl, C1-C5 alkoxy, C1-C5 alkoxycarbonyl, linear C1-C5 alkylcarbonyloxy or branched, C1-C3-alkoxy-C1-C3 alkoxy, C1-C3 alkylsulfanyl, C2-C5 alkenyloxy, formyl, pyridyl, naphthyl, thiazolyl (the thiazole ring is optionally substituted with one or more C1-C3 alkyl), thiophenyl (the thiophene ring is optionally substituted with one or more halogen), isoxazolyl (the isoxazol ring is optionally substituted with one or more C1-C3 alkyl), 1,3-dioxolanyl (the 1,3-dioxolane ring is optionally substituted with one or more C1-C3 alkyl), 1,3-dioxanyl, oxyranyl and tetrahydropyranyl； a C2-C6 alkenyl group optionally substituted with phenyl； a linear or branched C2-C6 alkynyl group； a C3-C6 dienyl group； or a group - (CH2) p-phenyl (p is 0, 1, 2 or 3 and the phenyl ring is optionally substituted with one or more substituents selected from the group consisting of halogen, cyano, a linear C1-C5 alkyl or branched, C1-C3 alkoxy, C1-C3 alkoxycarbonyl, C1-C3 trifluoroalkyl, C1-C3 trifluoroalkoxy and C1-C3 alkylsulfonyl), R2 is a linear or branched C1-C6 alkyl group, R3 is hydrogen； a linear or branched C1-C6 alkyl group, optionally substituted with one or more substituents selected from the group consisting of hydroxy, C1-C5 alkoxy, C1-C3 alkylcarbonyloxy, cyano, morpholinyl and mono-, di- or trialkylamino-C1- C3； a halogen group； a cyano group； a formal group； a C1-C3 alkylsulfanyl group； a C1-C3 alkylsulfonyl group； or a C1-C3 alkylsulfinyl group, R4 is hydrogen； a linear or branched C1-C6 alkyl group, optionally substituted with one or more substituents selected from the group consisting of halogen, hydroxy, cyano, amino, C1-C5 alkylamino, C3-C6 cycloalkylamino, phenylamino (the phenyl ring is optionally substituted wi
