The present invention provides compounds of formula (4), and their pharmaceutically acceptable salts and solvates, which are useful as inhibitors of the Hepatitis C virus (HCV) polymerase enzyme and are also useful for the treatment of HCV infections in HCV-infected mammals. The present invention also provides pharmaceutical compositions comprising compounds of formula (4), their pharmaceutically acceptable salts and solvates. Furthermore, the present invention provides intermediate compounds and methods useful in the preparation of compounds of formula (4). ##STR00001##