A compound of Formula (I) or a pharmaceutically acceptable salt thereof: ** Formula ** wherein: each R is independently H or C (O) CH3； R1 and R2 are independently selected from the group consisting of: H, C1-6 alkyl, C2-6 alkenyl, C1-6 alkoxy, - (CH2) n-cyclopropyl and - (CH2) n-cyclobutyl where n is 0, 1, 2, 3 or 4； or R1 and R2 can be attached together with the nitrogen atom to which they are attached to form azetidine, pyrrolidine, piperidine or isoxazolidine, said C1-6 alkyl, C2-6 alkenyl, C1-6 alkoxy, - (CH2) n-cyclopropyl being and - (CH2) n-cyclobutyl, azetidine, pyrrolidine, piperidine and isoxazolidine optionally mono-substituted with fluoro, hydroxy or methyl； R3 is selected from the group consisting of: C1-8alkyl, C2-8alkenyl, C2-8alkynyl, C3-6cycloalkyl, aryl and heteroaryl, each optionally mono-substituted with fluoro or OH； R4 is selected from the group consisting of: H, F, C1-8alkyl, C2-8alkenyl, C2-8alkynyl, each, other than H or F, optionally mono-substituted with fluoro or OH； or R3 and R4 and the carbon atom to which they are attached, can be attached to form vinyl or a 3- to 7-membered carbocyclic or heterocyclic ring, said 3- to 7-membered carbocyclic or heterocyclic ring containing, optionally, a double bond and being optionally mono-substituted with fluoro or OH； or a compound of Formula (Ia) or a pharmaceutically acceptable salt thereof: ** Formula ** in which: R1 and R2 are independently selected from the group consisting of: H, C1-6 alkyl and cyclopropylmethyl； or R1 and R2 can be attached together with the nitrogen atom to which they are attached to form azetidine or pyrrolidine, said C1-6 alkyl, cyclopropylmethyl, azetidine or pyrrolidine being optionally substituted with 1 to 3 substituents selected from fluoro and methyl； R3 is selected from the group consisting of: C1-8 alkyl, C2-8 alkenyl, C2-8 alkynyl, and C3-6 cycloalkyl, each optionally substituted with 1 to 3 substituents selected from fluoro and OH； and R4 is selected from the g
