The present invention relates to a compound of formula (I) or a pharmaceutically acceptable salt thereof, wherein R1 is selected from fluoro, methoxy and ethoxy, each R2 is independently selected from hydrogen, fluorine and methyl, n is O, 1, 2, 3, 4 or 5, R3 is selected from hydrogen, fluorine, amino, hydroxy, and a five or six membered substituted or unsubstituted ring system which may be aromatic or aliphatic, comprising 1 or 2 heteroatoms selected from the group consisting of nitrogen and oxygen, R4 is hydrogen or methyl, for use as a therapeutically active substance.
