Four millimeters is a unit of measurement in the metric system, equivalent to one-fourth of a centimeter. It is commonly used to measure small lengths, such as the thickness of a credit card or the diameter of a small screw. In everyday terms, 4 millimeters is about the width of a large paperclip or the thickness of a typical smartphone case. This measurement is often used in fields like engineering, manufacturing, and medicine, where precision is crucial for various applications, including 3D printing and surgical instruments.