The present invention provides compounds of formula (I)wherein R1, R2, R3and R4are as defined in the specification, and pharmaceutically acceptable salts thereof, as well as processes for their preparation, pharmaceutical compositions containing them and their use in therapy.本發明係提供式(I)化合物其中R1、R2、R3與R4係如說明書中定義,及其醫藥上可接受之鹽類,以及其製備方法、包含其之醫藥組成物,以及其用於治療之用途。