Four millimeters is equal to 5/32 inch, 0.4 centimeters, or 0.004 meters. There are 25.4 millimeters in one inch. Converting millimeters to inches involves multiplying the number of millimeters by 0.03937. For example, 8-gauge steel is approximately 4 millimeters thick.
The millimeter is a measure of length in the metric system, which uses base 10 for simplicity and is also known as the International System of Units. The metric system originated in France around 1790.
The United States attempted to move to the metric system with the Metric Conversion Act of 1975 which called for a voluntary conversion. However, the movement stalled.