The term "big 4" is being used to refer to a group of influential bands in various genres of metal and rock music. However, the specific bands that make up the "big 4" are unclear, with different outlets suggesting different genres and time periods, such as 80s metal, nu-metal, and 70s rock. The concept of the "big 4" appears to have originated in the context of thrash metal, but its definition and application are now being debated and expanded.