The invention relates to compounds of formula (I) wherein the substituents X1,R1, R2, R3 and R4 have the meaning as set forth and explained in thedescription of the invention, to processes for the preparation of thesecompounds, pharmaceutical compositions containing same, the use thereofoptionally in combination with one or more other pharmaceutically activecompounds for the therapy of a disease which responds to an inhibition ofprotein kinase activity, and a method for the treatment of such a disease.