A compound of formula I': wherein: R 4 is selected from aryl, heterocycle and heteroaryl, wherein any aryl, heterocycle and heteroaryl of R 4 is optionally substituted with one or more groups each independently selected from halo, (C 1 -C 6)alkyl, (C 2 -C 6)alkenyl, (C 1 -C 6)haloalkyl, (C 3 -C 7)cycloalkyl, -(C 1 -C 6)alkyl-(C 3 -C 7)cycloalkyl, -OH, -O(C 1 -C 6)alkyl, -SH, -S(C 1 -C 6)alkyl, NH 2, -NH(C 1 -C 6)alkyl and -N((C 1 -C 6)alkyl) 2, wherein (C 1 -C 6)alkyl is optionally substituted with hydroxy, -O(C 1 -C 6)alkyl, cyano or oxo; A is phenyl, monocyclic heteroaryl or monocyclic heterocycle, wherein any phenyl, monocyclic heteroaryl or monocyclic heterocycle of A is optionally substituted with one or more Z 1a groups, and B is aryl, heteroaryl or heterocycle, wherein any aryl, heteroaryl or heterocycle of B is optionally substituted with one or more Z 1b groups; or A and B together form a bicyclic aryl, bicyclic heteroaryl or bicyclic heterocycle, wherein bicyclic aryl, bicyclic heteroaryl or bicyclic heterocycle is optionally substituted with one or more Z 1b groups; each Z 1a is independently selected from halo, (C 1 -C 3)alkyl, (C 2 -C 3)alkenyl, (C 2 -C 3)alkynyl, (C 1 -C 3)haloalkyl, (C 3 -C 7)carbocycle, heterocycle, -O(C 1 -C 3)alkyl, -O(C 2 -C 3)alkenyl, -O(C 2 -C 3)alkynyl, -NR c R d, -NR a C R a, -C OR b and -C NR c R d, wherein any (C 3 -C 7)carbocycle and heterocycle of Z 1a is optionally substituted with one or more halogen or (C 1 -C 6)alkyl; each Z 1b is independently selected from halo, CN, (C 1 -C 6)alkyl, (C 2 -C 6)alkenyl, (C 2 -C 6)alkynyl, (C 1 -C 6)haloalkyl, (C 3 -C 7)carbocycle, heteroaryl, heterocycle, aryl(C 1 -C 6)alkyl-, -OH, -O(C 1 -C 6)alkyl, -O(C 2 -C 6)alkenyl, -O(C 2 -C 6)alkynyl,-NR c R d, -NR a C R a, -C OR b and -C NR c R d, wherein any (C 3 -C 7)carbocycle and heterocycle of Z 1b is optionally substituted with one or more halogen or (C 1 -C 6)alkyl; and R a, R b, R c and R d are each independently H or (C 1 -C 6)alkyl; or a salt thereof. R 4 is selected from aryl, heterocycle and heteroaryl, wherein any aryl, heterocycle and heteroaryl of R 4 is optionally substituted with one or more groups each independently selected from halo, (C 1 -C 6)alkyl, (C 2 -C 6)alkenyl, (C 1 -C 6)haloalkyl, (C 3 -C 7)cycloalkyl, -(C 1 -C 6)alkyl-(C 3 -C 7)cycloalkyl, -OH, -O(C 1 -C 6)alkyl, -SH, -S(C 1 -C 6)alkyl, NH 2, -NH(C 1 -C 6)alkyl and -N((C 1 -C 6)alkyl) 2, wherein (C 1 -C 6)alkyl is optionally substituted with hydroxy, -O(C 1 -C 6)alkyl, cyano or oxo; A is phenyl, monocyclic heteroaryl or monocyclic heterocycle, wherein any phenyl, monocyclic heteroaryl or monocyclic heterocycle of A is optionally substituted with one or more Z 1a groups, and B is aryl, heteroaryl or heterocycle, wherein any aryl, heteroaryl or heterocycle of B is optionally substituted with one or more Z 1b groups; or A and B together form a bicyclic aryl, bicyclic heteroaryl or bicyclic heterocycle, wherein bicyclic aryl, bicyclic heteroaryl or bicyclic heterocycle is optionally substituted with one or more Z 1b groups; each Z 1a is independently selected from halo, (C 1 -C 3)alkyl, (C 2 -C 3)alkenyl, (C 2 -C 3)alkynyl, (C 1 -C 3)haloalkyl, (C 3 -C 7)carbocycle, heterocycle, -O(C 1 -C 3)alkyl, -O(C 2 -C 3)alkenyl, -O(C 2 -C 3)alkynyl, -NR c R d, -NR a C R a, -C OR b and -C NR c R d, wherein any (C 3 -C 7)carbocycle and heterocycle of Z 1a is optionally substituted with one or more halogen or (C 1 -C 6)alkyl; each Z 1b is independently selected from halo, CN, (C 1 -C 6)alkyl, (C 2 -C 6)alkenyl, (C 2 -C 6)alkynyl, (C 1 -C 6)haloalkyl, (C 3 -C 7)carbocycle, heteroaryl, heterocycle, aryl(C 1 -C 6)alkyl-, -OH, -O(C 1 -C 6)alkyl, -O(C 2 -C 6)alkenyl, -O(C 2 -C 6)alkynyl,-NR c R d, -NR a C R a, -C OR b and -C NR c R d, wherein any (C 3 -C 7)carbocycle and heterocycle of Z 1b is optionally substituted with one or more halogen or (C 1 -C 6)alkyl; and R a, R b, R c and R d are each independently H or (C 1 -C 6)alkyl;