A compound of Formula I or a pharmaceutically acceptable salt thereof ** Formula ** wherein: R1 is selected from the group consisting of: - H, halogen, CN, (CH2) nNRaRa, C (O) NRaRa, C (O ) ORb, NRbC (O) Rb, (CH2) mORb, - C1-C6 alkyl optionally substituted with heterocycloalkyl, - heterocycloalkyl, - C1-C6 alkynyl optionally substituted with OH, C1-C3 alkoxy, NRaRa or heterocycloalkyl, - (CH2) nRc wherein Rc is optionally substituted with one or more substituents selected from the group consisting of C1-C3 alkoxy, C1-C3 alkyl, NRaRa and halogen, and - (CH2) nRd where Rd is optionally substituted with one or more selected substituents from the group consisting of C1-C3 alkoxy, C1-C3 alkyl, NRaRa and halogen； R2 is selected from the group consisting of: - H, halogen, CN, C (O) NRaRa, - C1-C6 alkyl, - heterocycloalkyl, - (CH2) nRc wherein Rc is optionally substituted with one or more substituents selected from the group consisting of C1-C3 alkoxy, (CH2) nNRaC (O) ORa, (CH2) nNRaRa, halogen, C1-C4 alkyl optionally substituted with one to three F, and heterocycloalkyl optionally substituted with C (O) ORb, and - (CH2) nRd wherein Rd is optionally substituted with one or more substituents selected from the group consisting of C1-C4 alkyl, C1-C3 alkoxy, halogen, heterocycloalkyl and (CH2) nNRaRa； R3 is selected from the group consisting of: - C1-C6 alkyl optionally substituted with Rc wherein Rc is optionally substituted with halogen, - heterocycloalkyl optionally substituted with one or more substituents selected from the group consisting of halogen and C (O) ORb in where Rb is optionally substituted with phenyl, -cycloalkyl, -Rc optionally substituted with one or more substituents selected from the group consisting of C1-C3 alkoxy, NRaRa, NO2, halogen, and C1-C4 alkyl optionally substituted with one to three F , and - Rd optionally substituted with one or more substituents selected from the group consisting of C1-C4 alkyl, C1-C3 alkoxy, halogen and phenyl； R4 is H, C1-C6
