Compounds of formula (I) and salts thereof: wherein R1, R2, R3, R4 are defined herein. Compounds of formula (I) and salts thereof have been found to inhibit the binding of the BET family of bromodomain proteins to, for example, acetylated lysine residues and thus may have use in therapy, for example in the treatment of autoimmune and inflammatory diseases, such as rheumatoid arthritis; and cancers.