A compound of formula or a salt thereof wherein: R 1 is -C 1-3 alkyl or cyclopropyl; R 2 is -C 0-4 alkyl-heterocyclyl, -CH 2 CH -heterocyclyl or -(CH 2) p O-heterocyclyl wherein each heterocyclyl is optionally substituted by one, two or three R 5 groups which may be the same or different and wherein at each occurrence the term heterocyclyl refers to a non-aromatic heterocyclic monocyclic or bicyclic ring system containing 4, 5, 6, 7, 8, 9 or 10 ring member atoms, including one heteroatom and optionally containing a further heteroatom selected from nitrogen, oxygen or sulphur; R 3 is -H, -C 1-4 alkyl, cyclopropyl, fluoro, chloro, -CH 2 F, -C 0-3 alkylOR 10 or C 0-3 alkylCN; R 4 is phenyl or a heteroaryl group wherein each are optionally substituted by one, two or three R 6 groups which may be the same or different, wherein the term heteroaryl refers to a monocyclic or bicyclic group having 5, 6, 8, 9, 10 or 11 member atoms, including 1, 2 or 3 heteroatoms independently selected from nitrogen, sulphur and oxygen, wherein at least a portion of the group is aromatic; each R 5 is independently halo, -C 1-4 alkyl, C3-4cycloalkyl, -CH 2 CF 3, -CH 2 CHF 2, - CH 2 CH 2 F, -OCH 2 CH 2 OR 8, -C 0-3 alkylOR 8, -C 0-3 alkylNR 11 R 12, -NHCH 2 CH 2 OR 8, -NHCO 2 R 8, oxo, -CO 2 R 8, -C R 8 or -C NR 11 R 12; each R 6 is independently halo, -C 1-4 alkyl, -C 0-3 alkylOR 7, oxo, -CO 2 R 8, -C 0-3 alkylNR 16 R 17, -C 0-3 alkyl-CONR 16 R 17, -CN, -SO 2 -C 1-3 alkyl or -SO 2 NR 16 R 17; R 7 is -H, -C 1-3 alkyl, -C 2-3 alkylNR 16 R 17 or -C 2-3 alkylOH; R 8 is -H or -C 1-4 alkyl; R 10 is -H, -C 1-3 alkyl, -C 2-3 alkylNR 14 R 15 or -C 2-3 alkylOH; R 11 and R 12 are each independently selected from -H and -C 1-3 alkyl; R 14 and R 15 are each independently selected from -H and -C 1-3 alkyl; R 16 and R 17 are each independently selected from -H and -C 1-3 alkyl; and p is an integer selected from 2, 3 or 4. R 1 is -C 1-3 alkyl or cyclopropyl; R 2 is -C 0-4 alkyl-heterocyclyl, -CH 2 CH -heterocyclyl or -(CH 2) p O-heterocyclyl wherein each heterocyclyl is optionally substituted by one, two or three R 5 groups which may be the same or different and wherein at each occurrence the term heterocyclyl refers to a non-aromatic heterocyclic monocyclic or bicyclic ring system containing 4, 5, 6, 7, 8, 9 or 10 ring member atoms, including one heteroatom and optionally containing a further heteroatom selected from nitrogen, oxygen or sulphur; R 3 is -H, -C 1-4 alkyl, cyclopropyl, fluoro, chloro, -CH 2 F, -C 0-3 alkylOR 10 or C 0-3 alkylCN; R 4 is phenyl or a heteroaryl group wherein each are optionally substituted by one, two or three R 6 groups which may be the same or different, wherein the term heteroaryl refers to a monocyclic or bicyclic group having 5, 6, 8, 9, 10 or 11 member atoms, including 1, 2 or 3 heteroatoms independently selected from nitrogen, sulphur and oxygen, wherein at least a portion of the group is aromatic; each R 5 is independently halo, -C 1-4 alkyl, C3-4cycloalkyl, -CH 2 CF 3, -CH 2 CHF 2, - CH 2 CH 2 F, -OCH 2 CH 2 OR 8, -C 0-3 alkylOR 8, -C 0-3 alkylNR 11 R 12, -NHCH 2 CH 2 OR 8, -NHCO 2 R 8, oxo, -CO 2 R 8, -C R 8 or -C NR 11 R 12; each R 6 is independently halo, -C 1-4 alkyl, -C 0-3 alkylOR 7, oxo, -CO 2 R 8, -C 0-3 alkylNR 16 R 17, -C 0-3 alkyl-CONR 16 R 17, -CN, -SO 2 -C 1-3 alkyl or -SO 2 NR 16 R 17; R 7 is -H, -C 1-3 alkyl, -C 2-3 alkylNR 16 R 17 or -C 2-3 alkylOH; R 8 is -H or -C 1-4 alkyl; R 10 is -H, -C 1-3 alkyl, -C 2-3 alkylNR 14 R 15 or -C 2-3 alkylOH; R 11 and R 12 are each independently selected from -H and -C 1-3 alkyl; R 14 and R 15 are each independently selected from -H and -C 1-3 alkyl; R 16 and R 17 are each independently selected from -H and -C 1-3 alkyl; and p is an integer selected from 2, 3 or 4.