A compound of formula or a salt thereof wherein R 1 is C 1-3 alkyl or cyclopropyl; R2 is -CH3, C 2-6 alkyl optionally substituted by up to five fluoro, -C 2-6 alkylOR 5, -C2- 6 alkylNR 5 R 6, - mSO 2 C 1-3 alkyl, -(CH 2) m C NR 5 R 6, -(CH 2) m CN, -(CH 2) m CO 2 R 5, - (CH 2) m NHCO 2 C(CH 3) 3 or -(CH 2) n C 5-6 heteroaryl wherein C 5-6 heteroaryl is optionally substituted by one or two substituents independently selected from halo, C 1-4 alkyl, C 3-4 cycloalkyl and -OR 5; R 3 is a) phenyl (which may be unsubstituted or substituted by one, two or three R 9 groups which may be the same or different); b) a C 5-6 heteroaryl group (which may be unsubstituted or substituted by C 1-3 alkyl, C3-4cycloalkyl, C 1-3 alkoxy or halo); c) a C 9-11 heteroaryl group (which may be unsubsituted or substituted by one, two or three groups independently selected from -C 1-3 alkylR 10, -OCH 3, -OCHF 2, -OCF 3, -OC 2-3 alkylR 10, halo, oxo and -CN); or d) -(CH 2) q -phenyl; R 4 is -H, C 1-4 alkyl, cyclopropyl, -CH 2 OR 11 or -CH 2 CH 2 OR 11; R 5 and R 6 are each independently selected from -H, C 1-3 alkyl and C 2-4 alkylOC 0-3 alkyl; R 9 is -NR 12 R 13, fluoro, -CN, -CH 2 CN, -CO 2 R 11, -C C 1-3 alkyl, -OH, -OCHF 2, -OCF 3, -O-C 2-6 alkylR 10, -OCH 3, -OCH 2 CH 2 NR 12 R 13, -C 1-6 alkylR 10, -OC 4-7 heterocyclyl, -OCH 2 C 4-7 heterocyclyl,-CH 2 C 4-7 heterocyclyl, -CH 2 CH 2 C 4-7 heterocyclyl, -NHC R 11, -SO 2 R 11 or -SOR 11; R 10 is -H, -OR 11 or -NR 12 R 13; R 11 is -H or C 1-3 alkyl; R 12 and R 13 are each independently selected from -H and C 1-3 alkyl; or R 12 and R 13 may join together with the nitrogen to which they are attached, to form a C 4-7 heterocyclyl optionally containing a further heteroatom selected from nitrogen, oxygen and sulphur and optionally substituted by one or two substituents independently selected from C 1-3 alkyl, -OH and fluoro; m is an integer selected from 0, 1, 2, 3 and 4; n is an integer selected from 2, 3 and 4; and q is an integer selected from 1 and 2. R 1 is C 1-3 alkyl or cyclopropyl; R2 is -CH3, C 2-6 alkyl optionally substituted by up to five fluoro, -C 2-6 alkylOR 5, -C2- 6 alkylNR 5 R 6, - mSO 2 C 1-3 alkyl, -(CH 2) m C NR 5 R 6, -(CH 2) m CN, -(CH 2) m CO 2 R 5, - (CH 2) m NHCO 2 C(CH 3) 3 or -(CH 2) n C 5-6 heteroaryl wherein C 5-6 heteroaryl is optionally substituted by one or two substituents independently selected from halo, C 1-4 alkyl, C 3-4 cycloalkyl and -OR 5; R 3 is a) phenyl (which may be unsubstituted or substituted by one, two or three R 9 groups which may be the same or different); b) a C 5-6 heteroaryl group (which may be unsubstituted or substituted by C 1-3 alkyl, C3-4cycloalkyl, C 1-3 alkoxy or halo); c) a C 9-11 heteroaryl group (which may be unsubsituted or substituted by one, two or three groups independently selected from -C 1-3 alkylR 10, -OCH 3, -OCHF 2, -OCF 3, -OC 2-3 alkylR 10, halo, oxo and -CN); or d) -(CH 2) q -phenyl; R 4 is -H, C 1-4 alkyl, cyclopropyl, -CH 2 OR 11 or -CH 2 CH 2 OR 11; R 5 and R 6 are each independently selected from -H, C 1-3 alkyl and C 2-4 alkylOC 0-3 alkyl; R 9 is -NR 12 R 13, fluoro, -CN, -CH 2 CN, -CO 2 R 11, -C C 1-3 alkyl, -OH, -OCHF 2, -OCF 3, -O-C 2-6 alkylR 10, -OCH 3, -OCH 2 CH 2 NR 12 R 13, -C 1-6 alkylR 10, -OC 4-7 heterocyclyl, -OCH 2 C 4-7 heterocyclyl,-CH 2 C 4-7 heterocyclyl, -CH 2 CH 2 C 4-7 heterocyclyl, -NHC R 11, -SO 2 R 11 or -SOR 11; R 10 is -H, -OR 11 or -NR 12 R 13; R 11 is -H or C 1-3 alkyl; R 12 and R 13 are each independently selected from -H and C 1-3 alkyl; or R 12 and R 13 may join together with the nitrogen to which they are attached, to form a C 4-7 heterocyclyl optionally containing a further heteroatom selected from nitrogen, oxygen and sulphur and optionally substituted by one or two substituents independently selected from C 1-3 alkyl, -OH and fluoro; m is an integer selected from 0, 1, 2, 3 and 4; n is an integer selected from 2, 3 and 4; and q is an integer selected from 1 and 2.