A compound of formula; or a pharmaceutically acceptable salt thereof, wherein R 1 represents a 5-7 membered non-aromatic hydrocarbon cyclic group optionally and independently substituted with 1-4 substituent(s) selected from the group consisting of a halogen atom, a cyano group, a hydroxyl group, an oxo group, an ethylenedioxy group, a C 1-6 alkyl group, a C 1-6 alkoxy group, a C 1-6 hydroxyalkyl group, -C OH, -CONH 2, a NHR 5 group, a NR 5 R 6 group and -R a -R b; wherein R a represents a single bond or -CH 2 -; wherein R b represents a 4-8 membered non-aromatic heterocyclic group, a C 6-10 aryl group or a 5-7 membered heteroaryl group, optionally and independently substituted with 1-4 substituent(s) selected from the group consisting of a halogen atom and a C 1-6 alkyl group; wherein R 5 and R 6 are independently selected from a C 1-6 alkyl group, a C 1-6 alkoxy group, a C 1-6 hydroxyalkyl group or a 6-membered non-aromatic heterocyclic group wherein two or more positions on R 1 are optionally bridged by a group -X-wherein X is O, CH 2, CH 2 -CH 2, NR 7, CH 2 -CH 2 -CH 2, CH 2 -CH(CH 2 -)-CH 2 or N(R 7)-CH(CH 2 -)CH 2 to form a bicyclic or tricyclic ring system, wherein R 7 is independently selected from hydrogen or a C 1-6 alkyl group and wherein said bridge may be optionally and independently substituted with one or more of a C 1-6 alkyl group, a cyano group, CO 2 NH 2, a C 1-6 hydroxyalkyl group, an oxo group, a hydroxy group, a C 1-6 alkylamino group or a 6-membered non-aromatic heterocyclic group wherein R 2 represents a hydrogen atom, a C 1-6 alkyl group optionally substituted with a 4-7 membered non-aromatic heterocyclic group or a C 1-6 haloalkyl group R 3 represents a hydrogen atom or a C 1-6 alkyl group and R 4 represents a hydrogen atom or a C 1-6 alkyl group. wherein R 1 represents a 5-7 membered non-aromatic hydrocarbon cyclic group optionally and independently substituted with 1-4 substituent(s) selected from the group consisting of a halogen atom, a cyano group, a hydroxyl group, an oxo group, an ethylenedioxy group, a C 1-6 alkyl group, a C 1-6 alkoxy group, a C 1-6 hydroxyalkyl group, -C OH, -CONH 2, a NHR 5 group, a NR 5 R 6 group and -R a -R b; wherein R a represents a single bond or -CH 2 -; wherein R b represents a 4-8 membered non-aromatic heterocyclic group, a C 6-10 aryl group or a 5-7 membered heteroaryl group, optionally and independently substituted with 1-4 substituent(s) selected from the group consisting of a halogen atom and a C 1-6 alkyl group; wherein R 5 and R 6 are independently selected from a C 1-6 alkyl group, a C 1-6 alkoxy group, a C 1-6 hydroxyalkyl group or a 6-membered non-aromatic heterocyclic group wherein two or more positions on R 1 are optionally bridged by a group -X-wherein X is O, CH 2, CH 2 -CH 2, NR 7, CH 2 -CH 2 -CH 2, CH 2 -CH(CH 2 -)-CH 2 or N(R 7)-CH(CH 2 -)CH 2 to form a bicyclic or tricyclic ring system, wherein R 7 is independently selected from hydrogen or a C 1-6 alkyl group and wherein said bridge may be optionally and independently substituted with one or more of a C 1-6 alkyl group, a cyano group, CO 2 NH 2, a C 1-6 hydroxyalkyl group, an oxo group, a hydroxy group, a C 1-6 alkylamino group or a 6-membered non-aromatic heterocyclic group wherein R 2 represents a hydrogen atom, a C 1-6 alkyl group optionally substituted with a 4-7 membered non-aromatic heterocyclic group or a C 1-6 haloalkyl group R 3 represents a hydrogen atom or a C 1-6 alkyl group and R 4 represents a hydrogen atom or a C 1-6 alkyl group.