A compound of formula: or a pharmaceutically acceptable salt thereof; wherein: R 1 is aryl or heteroaryl, wherein the aryl and the heteroaryl are optionally substituted with one, two, three, four, or five substituents independently selected from C 1 -C 3 alkyl, C 2 -C 4 alkenyl, C 2 -C 4 alkynyl, amino, aminoC 1 -C 3 alkyl, C 3 -C 4 cycloalkyl, halo, haloC 1 -C 3 alkyl, hydroxy, and hydroxyC 1 -C 3 alkyl; R 2 and R 3 are independently selected from hydrogen, C 1 -C 3 alkoxy, C 1 -C 3 alkyl, cyano, halo, haloC 1 -C 3 alkyl, and hydroxy; R 4 is selected from: wherein R a is hydrogen or C 1 -C 3 alkyl; and denotes the point of attachment to the parent molecular moiety; R 5 is -(C 1 -C 3 alkyl)-R 6 or -(C 1 -C 6 alkyl)NR c R d, wherein R 6 is selected from: a C 3 -C 6 cycloalkyl optionally substituted with NR c R d (C 1 -C 3 alkyl)-; and a five- to ten-membered monocyclic, bicyclic, or tricyclic fully saturated or fully unsaturated ring containing one nitrogen atom and optionally a second heteroatom selected from oxygen or nitrogen, wherein the ring contains zero to three double bonds and wherein the ring is optionally substituted with one, two, or three groups independently selected from C 1 -C 3 alkoxy, Ci-C 3 alkoxyC 1 -C 3 alkyl, C 1 -C 3 alkyl, benzyl, halo, haloC 1 -C 3 alkyl, hydroxy, hydroxyC 1 -C 3 alkyl, and oxo; wherein R c and R d, together with the nitrogen atom to which they are attached, form a five-to ten-membered ring monocyclic or bicyclic ring optionally containing one additional heteroatom selected from nitrogen, oxygen, and sulfur, wherein the ring is optionally substituted with one, two, or three groups independently selected from C 1 -C 3 alkoxy, C 1 -C 3 alkoxyC 1 -C 3 alkyl, C 1 -C 3 alkyl, benzyl, halo, haloC 1 -C 3 alkyl, hydroxy, hydroxyC 1 -C 3 alkyl, and oxo; or one of R c and R d is selected from hydrogen and C 1 -C 3 alkyl and the other is selected from hydrogen, C 1 -C 3 alkyl, C 1 -C 3 alkoxycarbonyl, and C 1 -C 3 alkylcarbonyl. R 1 is aryl or heteroaryl, wherein the aryl and the heteroaryl are optionally substituted with one, two, three, four, or five substituents independently selected from C 1 -C 3 alkyl, C 2 -C 4 alkenyl, C 2 -C 4 alkynyl, amino, aminoC 1 -C 3 alkyl, C 3 -C 4 cycloalkyl, halo, haloC 1 -C 3 alkyl, hydroxy, and hydroxyC 1 -C 3 alkyl; R 2 and R 3 are independently selected from hydrogen, C 1 -C 3 alkoxy, C 1 -C 3 alkyl, cyano, halo, haloC 1 -C 3 alkyl, and hydroxy; R 4 is selected from: wherein R a is hydrogen or C 1 -C 3 alkyl; and denotes the point of attachment to the parent molecular moiety; R a is hydrogen or C 1 -C 3 alkyl; and denotes the point of attachment to the parent molecular moiety; R 5 is -(C 1 -C 3 alkyl)-R 6 or -(C 1 -C 6 alkyl)NR c R d, wherein R 6 is selected from: a C 3 -C 6 cycloalkyl optionally substituted with NR c R d (C 1 -C 3 alkyl)-; and a five- to ten-membered monocyclic, bicyclic, or tricyclic fully saturated or fully unsaturated ring containing one nitrogen atom and optionally a second heteroatom selected from oxygen or nitrogen, wherein the ring contains zero to three double bonds and wherein the ring is optionally substituted with one, two, or three groups independently selected from C 1 -C 3 alkoxy, Ci-C 3 alkoxyC 1 -C 3 alkyl, C 1 -C 3 alkyl, benzyl, halo, haloC 1 -C 3 alkyl, hydroxy, hydroxyC 1 -C 3 alkyl, and oxo; wherein R c and R d, together with the nitrogen atom to which they are attached, form a five-to ten-membered ring monocyclic or bicyclic ring optionally containing one additional heteroatom selected from nitrogen, oxygen, and sulfur, wherein the ring is optionally substituted with one, two, or three groups independently selected from C 1 -C 3 alkoxy, C 1 -C 3 alkoxyC 1 -C 3 alkyl, C 1 -C 3 alkyl, benzyl, halo, haloC 1 -C 3 alkyl, hydroxy, hydroxyC 1 -C 3 alkyl, and oxo; or one of R c and R d is selected from hydrogen and C 1 -C 3 alkyl and the other is selected from hydrogen, C 1 -C 3 alkyl, C 1 -C 3 alkoxycarbonyl, and C 1 -C 3 alkylcarbonyl. a C 3 -C 6 cycloalkyl optionally substituted with NR c R d (C 1 -C 3 alkyl)-; and a five- to ten-membered monocyclic, bicyclic, or tricyclic fully saturated or fully unsaturated ring containing one nitrogen atom and optionally a second heteroatom selected from oxygen or nitrogen, wherein the ring contains zero to three double bonds and wherein the ring is optionally substituted with one, two, or three groups independently selected from C 1 -C 3 alkoxy, Ci-C 3 alkoxyC 1 -C 3 alkyl, C 1 -C 3 alkyl, benzyl, halo, haloC 1 -C 3 alkyl, hydroxy, hydroxyC 1 -C 3 alkyl, and oxo; wherein R c and R d, together with the nitrogen atom to which they are attached, form a five-to ten-membered ring monocyclic or bicyclic ring optionally containing one additional heteroatom selected from nitrogen, oxygen, and sulfur, wherein the ring is optionally substituted with one, two, or three groups independently selected from C 1 -C 3 alkoxy, C 1 -C 3 alkoxyC 1 -C 3 alkyl, C 1 -C 3 alkyl, benzyl, halo, haloC 1 -C 3 alkyl, hydroxy, hydroxyC 1 -C 3 alkyl, and oxo; or one of R c and R d is selected from hydrogen and C 1 -C 3 alkyl and the other is selected from hydrogen, C 1 -C 3 alkyl, C 1 -C 3 alkoxycarbonyl, and C 1 -C 3 alkylcarbonyl.