A compound of formula or a salt thereof wherein: R 1 is -C 1-3 alkyl or cyclopropyl; 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; 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 up to five fluoro, -C 2-6 alkylOR 13, -C 2 - 6 alkylNR 11 R 12, -(CH 2) m SO 2 C 1-3 alkyl, -(CH 2) m SO 2 NR 11 R 12, -(CH 2) m C NR 11 R 12, -(CH 2) m CN,-(CH 2) m CO 2 R 13, -(CH 2) m NHCO 2 C 1-4 alkyl -(CH 2) m NHC C 1-4 alkyl or -(CH 2) n heteroaryl wherein heteroaryl is optionally substituted by one or two R 14 groups which may be the same or different; R 3 is phenyl optionally substituted with one, two or three R 7 groups which may be the same or different; R 4 is -C 1-3 alkyl, -CH 2 OR 6 or -CH 2 F; each R 5 is independently halo, -C 0-6 alkyl-R 8, -O-C 2-6 alkyl-R 8, -CN or -SO 2 C 1-3 alkyl; R 6 is -H or C 1-3 alkyl; each R 7 is independently -halo, -C 1-4 alkyl, -C 0-3 alkyl-OR 10, -C 0-3 alkyl-NR 15 R 16, -C 0-3 alkyl-CONR 15 R 16, CN or -SO 2 R 17; R 8 is -H, -OR 10a, -NR 18 R 19 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 13, -C 0-3 alkyOR 13, -C 0-3 alkylNR 11 R 12, -NHCH 2 CH 2 OR 13, -NHCO 2 R 13, oxo,-C R 13, -C OR 13 or -C NR 11 R 12; R 10a is -H, -C 1-3 alkyl, -C 2-3 alkylNR 11 R 12 or -C 2-3 alkylOH; R 10 is -H, -C 1-3 alkyl, -C 2-3 alkylNR 15 R 16 or -C 2-3 alkylOH; R 11 and R 12 are each independently selected from -H and -C 1-3 alkyl; or R 11 and R 12 may join together with the nitrogen to which they are attached, to form a 4 to 7-membered heterocyclyl group optionally substituted by one or two substituents independently selected from -C 1-3 alkyl,-OH and F; R 13 is -H or C 1-4 alkyl; each R 14 is independently halo, C 1-4 alkyl, cyclopropyl, cyclobutyl or -OR 13; R 15 and R 16 are each independently selected from -H and -C 1-3 alkyl; or R 15 and R 16 may join together with the nitrogen to which they are attached, to form a 4 to 7-membered heterocyclyl group optionally substituted by one or two substituents independently selected from -C 1-3 alkyl,-OH and F; R 17 is -C 1-3 alkyl or -NR 15 R 16; R 18 and R 19 are each independently selected from -H, -C OC(CH 3) 3, -C 1-6 alkyl, cycloalkyl, heterocyclyl, -C 2-3 alkylNR 13 COC 1-3 alkyl, C 2-3 alkylNR 15 R 16 and -C 2-3 alkyl-O-C 1-3 alkyl wherein the -C 1-6 alkyl and cycloalkyl may be optionally substituted by one, two or three fluoro; or R 18 and R 19 may join together with the nitrogen to which they are attached, to form a 4 to 7-membered heterocyclyl group optionally substituted by one or two substituents independently selected from -C 1-3 alkyl,-OH and F; m is an integer selected from 2, 3 and 4; p is an integer selected from 2, 3 and 4; n is an integer selected from 0, 1, 2, 3 and 4. R 1 is -C 1-3 alkyl or cyclopropyl; 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; 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 up to five fluoro, -C 2-6 alkylOR 13, -C 2 - 6 alkylNR 11 R 12, -(CH 2) m SO 2 C 1-3 alkyl, -(CH 2) m SO 2 NR 11 R 12, -(CH 2) m C NR 11 R 12, -(CH 2) m CN,-(CH 2) m CO 2 R 13, -(CH 2) m NHCO 2 C 1-4 alkyl -(CH 2) m NHC C 1-4 alkyl or -(CH 2) n heteroaryl wherein heteroaryl is optionally substituted by one or two R 14 groups which may be the same or different; R 3 is phenyl optionally substituted with one, two or three R 7 groups which may be the same or different; R 4 is -C 1-3 alkyl, -CH 2 OR 6 or -CH 2 F; each R 5 is independently halo, -C 0-6 alkyl-R 8, -O-C 2-6 alkyl-R 8, -CN or -SO 2 C 1-3 alkyl; R 6 is -H or C 1-3 alkyl; each R 7 is independently -halo, -C 1-4 alkyl, -C 0-3 alkyl-OR 10, -C 0-3 alkyl-NR 15 R 16, -C 0-3 alkyl-CONR 15 R 16, CN or -SO 2 R 17; R 8 is -H, -OR 10a, -NR 18 R 19 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 13, -C 0-3 alkyOR 13, -C 0-3 alkylNR 11 R 12, -NHCH 2 CH 2 OR 13, -NHCO 2 R 13, oxo,-C R 13, -C OR 13 or -C NR 11 R 12; R 10a is -H, -C 1-3 alkyl, -C 2-3 alkylNR 11 R 12 or -C 2-3 alkylOH; R 10 is -H, -C 1-3 alkyl, -C 2-3 alkylNR 15 R 16 or -C 2-3 alkylOH; R 11 and R 12 are each independently selected from -H and -C 1-3 alkyl; or R 11 and R 12 may join together with the nitrogen to which they are attached, to form a 4 to 7-membered heterocyclyl group optionally substituted by one or two substituents independently selected from -C 1-3 alkyl,-OH and F; R 13 is -H or C 1-4 alkyl; each R 14 is independently halo, C 1-4 alkyl, cyclopropyl, cyclobutyl or -OR 13; R 15 and R 16 are each independently selected from -H and -C 1-3 alkyl; or R 15 and R 16 may join together with the nitrogen to which they are attached, to form a 4 to 7-membered heterocyclyl group optionally substituted by one or two substituents independently selected from -C 1-3 alkyl,-OH and F;