A "four-high" mill is a type of rolling mill used in metalworking. It consists of four rolls: two outer rolls and two inner rolls. The outer rolls apply pressure to the metal, while the inner rolls support the outer ones, allowing for better control of the thickness and shape of the material being processed. This configuration is particularly effective for reducing the thickness of materials like steel and aluminum. The four-high design minimizes the distance between the rolls, which helps to produce a smoother finish and improves the efficiency of the rolling process compared to other mill types.