A compound of structural Formula: or a salt thereof, wherein: R 1 and R 2 are independently chosen from C 1-4 alkyl, C 1-4 haloalkyl, C 3-6 cycloalkyl, C 3-6 heterocycloalkyl, C 5-10 aryl, and 5-10 membered heteroaryl, any of which is optionally substituted with one or more R 5 groups, or R 1 and R 2, together with the sulfur to which they are both attached, form a 4, 5, 6, or 7-membered heterocycloalkyl ring which is optionally substituted with one or more R 5 groups; R 3 is chosen from hydrogen, C 1-6 alkyl, and C 1-6 haloalkyl; R 4 is chosen from C 5-10 aryl or 5-10 membered heteroaryl, either of which is optionally substituted with one or more R 6 groups; each R 5 is independently chosen from NR 8 R 9, halogen, cyano, hydroxy, oxo, alkyl, haloalkyl, C 3-6 cycloalkyl, 3-6 membered heterocycloalkyl, hydroxyalkyl, OR 8, NR 7 C R 8, NR 7 C OR 8, NR 7 C NR 8 R 9, C R 8, C OR 8, and C NR 8 R 9; each R 6 is independently chosen from NR 11 R 12, halogen, cyano, hydroxy, oxo, alkyl, haloalkyl, C 3-6 cycloalkyl, 3-6 membered heterocycloalkyl, hydroxyalkyl, OR 11, NR 10 C R 11, NR 10 C OR 11, NR 10 C NR 11 R 12, C R 11, C OR 11, and C NR 11 R 12; each R 7, R 8 and R 9 is independently chosen from hydrogen, C 1-4 alkyl, C 3-6 cycloalkyl, and 3-6 membered heterocycloalkyl, any of which is optionally substituted with halo, hydroxy, C 1-3 alkyl, C 1-3 haloalkyl and C 1-3 alkoxy; or any two of R 7, R 8 and R 9, together with the atom to which they are both attached can form a 3-7 membered cycloalkyl or heterocycloalkyl ring; and each R 10, R 11 and R 12 is independently chosen from hydrogen, C 1-4 alkyl, C 3-6 cycloalkyl, 3-6 membered heterocycloalkyl, any of which is optionally substituted with one or more groups chosen from halo, hydroxy and alkoxy; or any two of R 10, R 11 and R 12, together with the atom to which they are both attached, can form a 3-7 membered cycloalkyl or heterocycloalkyl ring. R 1 and R 2 are independently chosen from C 1-4 alkyl, C 1-4 haloalkyl, C 3-6 cycloalkyl, C 3-6 heterocycloalkyl, C 5-10 aryl, and 5-10 membered heteroaryl, any of which is optionally substituted with one or more R 5 groups, or R 1 and R 2, together with the sulfur to which they are both attached, form a 4, 5, 6, or 7-membered heterocycloalkyl ring which is optionally substituted with one or more R 5 groups; R 3 is chosen from hydrogen, C 1-6 alkyl, and C 1-6 haloalkyl; R 4 is chosen from C 5-10 aryl or 5-10 membered heteroaryl, either of which is optionally substituted with one or more R 6 groups; each R 5 is independently chosen from NR 8 R 9, halogen, cyano, hydroxy, oxo, alkyl, haloalkyl, C 3-6 cycloalkyl, 3-6 membered heterocycloalkyl, hydroxyalkyl, OR 8, NR 7 C R 8, NR 7 C OR 8, NR 7 C NR 8 R 9, C R 8, C OR 8, and C NR 8 R 9; each R 6 is independently chosen from NR 11 R 12, halogen, cyano, hydroxy, oxo, alkyl, haloalkyl, C 3-6 cycloalkyl, 3-6 membered heterocycloalkyl, hydroxyalkyl, OR 11, NR 10 C R 11, NR 10 C OR 11, NR 10 C NR 11 R 12, C R 11, C OR 11, and C NR 11 R 12; each R 7, R 8 and R 9 is independently chosen from hydrogen, C 1-4 alkyl, C 3-6 cycloalkyl, and 3-6 membered heterocycloalkyl, any of which is optionally substituted with halo, hydroxy, C 1-3 alkyl, C 1-3 haloalkyl and C 1-3 alkoxy; or any two of R 7, R 8 and R 9, together with the atom to which they are both attached can form a 3-7 membered cycloalkyl or heterocycloalkyl ring; and each R 10, R 11 and R 12 is independently chosen from hydrogen, C 1-4 alkyl, C 3-6 cycloalkyl, 3-6 membered heterocycloalkyl, any of which is optionally substituted with one or more groups chosen from halo, hydroxy and alkoxy; or any two of R 10, R 11 and R 12, together with the atom to which they are both attached, can form a 3-7 membered cycloalkyl or heterocycloalkyl ring.