A compound of the formula wherein R 1 is C 3-7 -alkyl, R 2 is selected from the group consisting of C 1-7 -alkyl, hydroxy-C 1-7 -alkyl, C 2-7 -alkenyl, C 3-7 -alkinyl, amino-C 1-7 -alkoxy-C 1-7 -alkyl, amino-C 1-7 -alkoxy-C 1-7 -alkoxy-C 1-7 -alkyl, halogen-C 1-7 -alkyl, C 3-7 -cycloalkyl-C 1-7 -alkyl and phenyl-C 1-7 -alkyl, wherein phenyl is unsubstituted or substituted by amino-C 1-7 -alkyl; R 3 is hydrogen; R 4 is selected from the group consisting of phenyl, said phenyl being unsubstituted or substituted by one or two groups selected from the group consisting of C 1-7 -alkyl, halogen, halogen-C 1-7 -alkyl, C 1-7 -alkoxy, hydroxy-C 1-7 -alkyl, amino-C 1-7 -alkyl, C 1-7 -alkyl-amino-C 1-7 -alkyl, di-C 1-7 -alkyl-amino-C 1-7 -alkyl, amino-C 2-7 -alkenyl, C 1-7 -alkyl-amino-C 2-7 -alkenyl, di-C 1-7 -alkyl-amino-C 2-7 -alkenyl, amino-C 2-7 -alkinyl, C 1-7 -alkyl-amino-C 2-7 -alkinyl, di-C 1-7 -alkyl-amino-C 2-7 -alkinyl, benzyloxycarbonylamino-C 1-7 -alkyl, amino-C 1-7 -alkoxy, amino-C 1-7 -alkoxy-C 1-7 -alkoxy, amino-C 1-7 -alkoxy-C 1-7 -alkyl, amino-C 1-7 -alkoxy-C 1-7 -alkoxy-C 1-7 -alkyl, C 1-7 -alkylsulfonyl, heterocyclylcarbonyl and phenyl-C 1-7 -alkyl, wherein phenyl is unsubstituted or substituted by C 1-7 -alkoxy or amino-C 1-7 -alkyl, or heteroaryl, said heteroaryl being a 5- or 6-membered aromatic ring containing one, two or three heteroatoms selected from N, O or S selected from the group consisting of imidazolyl, pyrazolyl, oxazolyl, thiazolyl, pyridyl, pyridazinyl and pyrimidinyl, and being unsubstituted or substituted by one or two groups selected from the group consisting of C 1-7 -alkyl, halogen, halogen-C 1-7 -alkyl, C 1-7 -alkoxy, hydroxy-C 1-7 -alkyl, amino-C 1-7 -alkyl, C 1-7 -alkyl-amino-C 1-7 -alkyl, di-C 1-7 -alkyl-amino-C 1-7 -alkyl, amino-C 2-7 -alkenyl, C 1-7 -alkyl-amino-C 2-7 -alkenyl, di-C 1-7 -alkyl-amino-C 2-7 -alkenyl, amino-C 2-7 -alkinyl, C 1-7 -alkyl-amino-C 2-7 -alkinyl, di-C 1-7 -alkyl-amino-C 2-7 -alkinyl, benzyloxycarbonylamino-C 1-7 -alkyl, amino-C 1-7 -alkoxy, amino-C 1-7 -alkoxy-C 1-7 -alkoxy, amino-C 1-7 -alkoxy-C 1-7 -alkyl, amino-C 1-7 -alkoxy-C 1-7 -alkoxy-C 1-7 -alkyl, C 1-7 -alkylsulfonyl, heterocyclylcarbonyl and phenyl-C 1-7 -alkyl, wherein phenyl is unsubstituted or substituted by C 1-7 -alkoxy or amino-C 1-7 -alkyl, or pharmaceutically acceptable salts thereof. R 1 is C 3-7 -alkyl, R 2 is selected from the group consisting of C 1-7 -alkyl, hydroxy-C 1-7 -alkyl, C 2-7 -alkenyl, C 3-7 -alkinyl, amino-C 1-7 -alkoxy-C 1-7 -alkyl, amino-C 1-7 -alkoxy-C 1-7 -alkoxy-C 1-7 -alkyl, halogen-C 1-7 -alkyl, C 3-7 -cycloalkyl-C 1-7 -alkyl and phenyl-C 1-7 -alkyl, wherein phenyl is unsubstituted or substituted by amino-C 1-7 -alkyl; R 3 is hydrogen; R 4 is selected from the group consisting of phenyl, said phenyl being unsubstituted or substituted by one or two groups selected from the group consisting of C 1-7 -alkyl, halogen, halogen-C 1-7 -alkyl, C 1-7 -alkoxy, hydroxy-C 1-7 -alkyl, amino-C 1-7 -alkyl, C 1-7 -alkyl-amino-C 1-7 -alkyl, di-C 1-7 -alkyl-amino-C 1-7 -alkyl, amino-C 2-7 -alkenyl, C 1-7 -alkyl-amino-C 2-7 -alkenyl, di-C 1-7 -alkyl-amino-C 2-7 -alkenyl, amino-C 2-7 -alkinyl, C 1-7 -alkyl-amino-C 2-7 -alkinyl, di-C 1-7 -alkyl-amino-C 2-7 -alkinyl, benzyloxycarbonylamino-C 1-7 -alkyl, amino-C 1-7 -alkoxy, amino-C 1-7 -alkoxy-C 1-7 -alkoxy, amino-C 1-7 -alkoxy-C 1-7 -alkyl, amino-C 1-7 -alkoxy-C 1-7 -alkoxy-C 1-7 -alkyl, C 1-7 -alkylsulfonyl, heterocyclylcarbonyl and phenyl-C 1-7 -alkyl, wherein phenyl is unsubstituted or substituted by C 1-7 -alkoxy or amino-C 1-7 -alkyl, or heteroaryl, said heteroaryl being a 5- or 6-membered aromatic ring containing one, two or three heteroatoms selected from N, O or S selected from the group consisting of imidazolyl, pyrazolyl, oxazolyl, thiazolyl, pyridyl, pyridazinyl and pyrimidinyl, and being unsubstituted or substituted by one or two groups selected from the group consisting of C 1-7 -alkyl, halogen, halogen-C 1-7 -alkyl, C 1-7 -alkoxy, hydroxy-C 1-7 -alkyl, amino-C 1-7 -alkyl, C 1-7 -alkyl-amino-C 1-7 -alkyl, di-C 1-7 -alkyl-amino-C 1-7 -alkyl, amino-C 2-7 -alkenyl, C 1-7 -alkyl-amino-C 2-7 -alkenyl, di-C 1-7 -alkyl-amino-C 2-7 -alkenyl, amino-C 2-7 -alkinyl, C 1-7 -alkyl-amino-C 2-7 -alkinyl, di-C 1-7 -alkyl-amino-C 2-7 -alkinyl, benzyloxycarbonylamino-C 1-7 -alkyl, amino-C 1-7 -alkoxy, amino-C 1-7 -alkoxy-C 1-7 -alkoxy, amino-C 1-7 -alkoxy-C 1-7 -alkyl, amino-C 1-7 -alkoxy-C 1-7 -alkoxy-C 1-7 -alkyl, C 1-7 -alkylsulfonyl, heterocyclylcarbonyl and phenyl-C 1-7 -alkyl, wherein phenyl is unsubstituted or substituted by C 1-7 -alkoxy or amino-C 1-7 -alkyl,