A compound of formula or a pharmaceutically acceptable salt thereof wherein: R 1 is methyl; R 2 is -C 0-3 alkyl-cycloalkyl, wherein the cycloalkyl group is optionally substituted with one, two or three R 5 groups which may be the same or different; or R 2 is -C 0-4 alkyl-heterocyclyl or -(CH 2) p O-heterocyclyl wherein each heterocyclyl is optionally substituted by one or two R 9 groups which may be the same or different; or R 2 is H, -CH 3, -C 2-6 alkyl optionally substituted by one, two, three, four or five fluoro, -C 2 - 6 alkylOR 6, -C 2-6 alkylNR 10a R 11a, -(CH 2) m SO 2 C 1-3 alkyl, -(CH 2) m SO 2 NR 10 R 11, -(CH 2) m C NR 10 R 11, - (CH 2) m CN, -(CH 2) m CO 2 R 6, -(CH 2) m NHCO 2 C 1-4 alkyl, -(CH 2) m NHC C 1-4 alkyl or -(CH 2) n heteroaryl wherein the heteroaryl is optionally substituted by one or two R 7 groups which may be the same or different; R 3 is H, -C 1-4 alkyl, cyclopropyl, -CH 2 F, -C 1-3 alkylOR 6 or -C 1-3 alkylCN; R 4 is phenyl or a heteroaryl group selected from the group consisting of pyridyl, indolyl and pyrrolopyridinyl, wherein each are optionally substituted by one, two or three R 7 groups which may be the same or different; each R 5 is independently halo, -C 0-6 alkyl-R 8, -O-C 2-6 alkyl-R 8, -OCH 2 phenyl, -CN or -SO 2 C 1-3 alkyl; R 6 is H or -C 1-4 alkyl; each R 7 is independently oxo, halo, -C 1-4 alkyl optionally substituted by one, two or three fluoro, -C 0-3 alkylOR 6, -OC 2-3 alkylOR 6, -C 0-3 alkylNR 10 R 11, -C 0-3 alkyl-CONR 10 R 11, -CN, -SO 2 -C 1-3 alkyl,-SO 2 NR 10 R 11 or-SO 2 phenyl optionally substituted by -C 1-4 alkyl; R 8 is H, -OR 6, -NR 10 R 11 or heteroaryl; each R 9 is idependently halo, -C 1-4 alkyl, cyclopropyl, cyclobutyl, -CH 2 CF 3, -CH 2 CHF 2, - CH 2 CH 2 F, -OCH 2 CH 2 OR 6, -C 0-3 alkylOR 6, -C 0-3 alkylNR 10 R 11, -NHCH 2 CH 2 OR 6, -NHCO 2 C 1-4 alkyl, oxo, - C R 6, -C OR 6 or -C NR 10 R 11; R 10 and R 11 are each independently selected from H and -C 1-3 alkyl; or R 10 and R 11 may join together with the nitrogen to which they are attached, to form a 4 to 7-membered heterocyclyl optionally substituted by one or two substituents independently selected from -C 1-3 alkyl optionally substituted with one, two or three fluorine atoms, -C 2-4 alkylOH, -OH and F; R 10a and R 11a are each independently selected from H and -C 1-3 alkyl; m is an integer selected from 2, 3 or 4; n is an integer selected from 0, 1, 2, 3 or 4; and p is an integer selected from 2, 3 or 4. R 1 is methyl; R 2 is -C 0-3 alkyl-cycloalkyl, wherein the cycloalkyl group is optionally substituted with one, two or three R 5 groups which may be the same or different; or R 2 is -C 0-4 alkyl-heterocyclyl or -(CH 2) p O-heterocyclyl wherein each heterocyclyl is optionally substituted by one or two R 9 groups which may be the same or different; or R 2 is H, -CH 3, -C 2-6 alkyl optionally substituted by one, two, three, four or five fluoro, -C 2 - 6 alkylOR 6, -C 2-6 alkylNR 10a R 11a, -(CH 2) m SO 2 C 1-3 alkyl, -(CH 2) m SO 2 NR 10 R 11, -(CH 2) m C NR 10 R 11, - (CH 2) m CN, -(CH 2) m CO 2 R 6, -(CH 2) m NHCO 2 C 1-4 alkyl, -(CH 2) m NHC C 1-4 alkyl or -(CH 2) n heteroaryl wherein the heteroaryl is optionally substituted by one or two R 7 groups which may be the same or different; R 3 is H, -C 1-4 alkyl, cyclopropyl, -CH 2 F, -C 1-3 alkylOR 6 or -C 1-3 alkylCN; R 4 is phenyl or a heteroaryl group selected from the group consisting of pyridyl, indolyl and pyrrolopyridinyl, wherein each are optionally substituted by one, two or three R 7 groups which may be the same or different; each R 5 is independently halo, -C 0-6 alkyl-R 8, -O-C 2-6 alkyl-R 8, -OCH 2 phenyl, -CN or -SO 2 C 1-3 alkyl; R 6 is H or -C 1-4 alkyl; each R 7 is independently oxo, halo, -C 1-4 alkyl optionally substituted by one, two or three fluoro, -C 0-3 alkylOR 6, -OC 2-3 alkylOR 6, -C 0-3 alkylNR 10 R 11, -C 0-3 alkyl-CONR 10 R 11, -CN, -SO 2 -C 1-3 alkyl,-SO 2 NR 10 R 11 or-SO 2 phenyl optionally substituted by -C 1-4 alkyl; R 8 is H, -OR 6, -NR 10 R 11 or heteroaryl; each R 9 is idependently halo, -C 1-4 alkyl, cyclopropyl, cyclobutyl, -CH 2 CF 3, -CH 2 CHF 2, - CH 2 CH 2 F, -OCH 2 CH 2 OR 6, -C 0-3 alkylOR 6, -C 0-3 alkylNR 10 R 11, -NHCH 2 CH 2 OR 6, -NHCO 2 C 1-4 alkyl, oxo, - C R 6, -C OR 6 or -C NR 10 R 11; R 10 and R 11 are each independently selected from H and -C 1-3 alkyl; or R 10 and R 11 may join together with the nitrogen to which they are attached, to form a 4 to 7-membered heterocyclyl optionally substituted by one or two substituents independently selected from -C 1-3 alkyl optionally substituted with one, two or three fluorine atoms, -C 2-4 alkylOH, -OH and F; R 10a and R 11a are each independently selected from H and -C 1-3 alkyl; m is an integer selected from 2, 3 or 4; n is an integer selected from 0, 1, 2, 3 or 4; and p is an integer selected from 2, 3 or 4.