Compounds of Formula I: and stereoisomers and pharmaceutically acceptable salts and solvates thereof in which R1, R2, R3 and R4 have the meanings given in the specification, are inhibitors of one or more JAK kinases and are useful in the treatment of JAK kinase-associated diseases and disorders, such as autoimmune diseases, inflammatory diseases, rejection of transplanted organs, tissues and cells, as well as hematologic disorders and malignancies and their co-morbidities.