The speed of wind is measured using an anemometer. An anemometer is a device that is specifically designed to measure the speed or velocity of wind.
Option 1, a barometer, is used to measure atmospheric pressure, not wind speed.
Option 2, a hygrometer, is used to measure the humidity or moisture content in the air, not wind speed.
Option 3, a thermometer, is used to measure the temperature, not wind speed.
Therefore, the correct option is 4 - anemometer. It is important to note that anemometers come in different types, such as cup anemometers, propeller anemometers, and sonic anemometers, each using different mechanisms to accurately measure wind speed.