A compound of the formula I: wherein: R 1 is selected from the group consisting of: phenyl, pyridyl, benzodioxolyl, dihydrobenzodioxinyl, indolyl, oxoisoindolinyl, pyrazolyl, and pyrimidinyl, which is substituted with one or more R 1a, R 1b and R 1c, wherein R 1a, R 1b and R 1c are independently selected from the group consisting of: (a) hydrogen, (b) hydroxy, (c) halogen, (d) C 1-6 alkyl, which is unsubstituted or substituted with substituents selected from the group consisting of: hydroxy, 1-3 fluoro, -OCH 3, - OCH 2 CH 2 OCH 3, -(C=O)-C 1-6 alkyl, -NH 2, -NH(C 1-6 alkyl), and -N(C 1-6 alkyl) 2, and -NH(C=O)(C 1-6 alkyl), (e) -O-C 1-6 alkyl, which is unsubstituted or substituted with substituents selected from the group consisting of: hydroxy, 1-3 fluoro, -OCH 3, - (C=O)-C 1-6 alkyl, -NH 2, -NH(C 1-6 alkyl), and -N(C 1-6 alkyl) 2, and - NH(C=O)(C 1 - 6 alkyl), (f) -O-C 3-6 alkenyl, (g) -NH 2, (h) -NH(C 1-6 alkyl), (i) -N(C 1-6 alkyl) 2, (j) -(C=O)NH 2, (k) -(C=O)NH(C 1-6 alkyl), (l) -(C=O)N(C 1-6 alkyl) 2, (m) -(C=O)-C 1-6 alkyl, (n) -(C=O)O-C 1-6 alkyl, (o) -O(C=O)O-C 1-6 alkyl, (p) -phenyl, (q) -SO 2 NH 2, (r) -SO 2 N(C 1-6 alkyl) 2, and (s) -NO 2; R 2 is hydrogen or methyl; R 3 is hydrogen or methyl; and R 4 is hydrogen or methyl; or a pharmaceutically acceptable salt thereof. R 1 is selected from the group consisting of: phenyl, pyridyl, benzodioxolyl, dihydrobenzodioxinyl, indolyl, oxoisoindolinyl, pyrazolyl, and pyrimidinyl, which is substituted with one or more R 1a, R 1b and R 1c, wherein R 1a, R 1b and R 1c are independently selected from the group consisting of: (a) hydrogen, (b) hydroxy, (c) halogen, (d) C 1-6 alkyl, which is unsubstituted or substituted with substituents selected from the group consisting of: hydroxy, 1-3 fluoro, -OCH 3, - OCH 2 CH 2 OCH 3, -(C=O)-C 1-6 alkyl, -NH 2, -NH(C 1-6 alkyl), and -N(C 1-6 alkyl) 2, and -NH(C=O)(C 1-6 alkyl), (e) -O-C 1-6 alkyl, which is unsubstituted or substituted with substituents selected from the group consisting of: hydroxy, 1-3 fluoro, -OCH 3, - (C=O)-C 1-6 alkyl, -NH 2, -NH(C 1-6 alkyl), and -N(C 1-6 alkyl) 2, and - NH(C=O)(C 1 - 6 alkyl), (f) -O-C 3-6 alkenyl, (g) -NH 2, (h) -NH(C 1-6 alkyl), (i) -N(C 1-6 alkyl) 2, (j) -(C=O)NH 2, (k) -(C=O)NH(C 1-6 alkyl), (l) -(C=O)N(C 1-6 alkyl) 2, (m) -(C=O)-C 1-6 alkyl, (n) -(C=O)O-C 1-6 alkyl, (o) -O(C=O)O-C 1-6 alkyl, (p) -phenyl, (q) -SO 2 NH 2, (r) -SO 2 N(C 1-6 alkyl) 2, and (s) -NO 2; (a) hydrogen, (b) hydroxy, (c) halogen, (d) C 1-6 alkyl, which is unsubstituted or substituted with substituents selected from the group consisting of: hydroxy, 1-3 fluoro, -OCH 3, - OCH 2 CH 2 OCH 3, -(C=O)-C 1-6 alkyl, -NH 2, -NH(C 1-6 alkyl), and -N(C 1-6 alkyl) 2, and -NH(C=O)(C 1-6 alkyl), (e) -O-C 1-6 alkyl, which is unsubstituted or substituted with substituents selected from the group consisting of: hydroxy, 1-3 fluoro, -OCH 3, - (C=O)-C 1-6 alkyl, -NH 2, -NH(C 1-6 alkyl), and -N(C 1-6 alkyl) 2, and - NH(C=O)(C 1 - 6 alkyl), (f) -O-C 3-6 alkenyl, (g) -NH 2, (h) -NH(C 1-6 alkyl), (i) -N(C 1-6 alkyl) 2, (j) -(C=O)NH 2, (k) -(C=O)NH(C 1-6 alkyl), (l) -(C=O)N(C 1-6 alkyl) 2, (m) -(C=O)-C 1-6 alkyl, (n) -(C=O)O-C 1-6 alkyl, (o) -O(C=O)O-C 1-6 alkyl, (p) -phenyl, (q) -SO 2 NH 2, (r) -SO 2 N(C 1-6 alkyl) 2, and (s) -NO 2; R 2 is hydrogen or methyl; R 3 is hydrogen or methyl; and R 4 is hydrogen or methyl; or a pharmaceutically acceptable salt thereof.