This invention relates to novel pyrazole derivatives of formula I wherein R1 to R4 are as defined in the summary and pharmaceutically acceptable salts and solvates thereof, methods to inhibit or modulate Human Immunodeficiency Virus (HIV) reverse transcriptase with compounds of formula I and pharmaceutical compositions containing of formula I admixed with at least one solvent, carrier or excipient. The compounds are useful for treating disorders in which HIV and genetically related viruses are implicated ##STR00001##