The correct answer is option 4, anemometer. An anemometer is a device used to measure the speed of wind. It consists of three or four cups attached to a central axis, which is mounted on a rotating spindle. When the wind blows, it causes the cups to rotate. By measuring the rate of rotation, the anemometer can determine the speed of the wind.

Option 1, the wind vane, is used to determine the direction from which the wind is blowing, not the speed. It typically consists of a pointer or a figure that aligns with the current wind direction.

Option 2, the wind rose, is a graphic representation that shows the distribution of wind direction and speed at a particular location over a given time period. It is a useful tool for studying and analyzing wind patterns, but it does not directly measure the speed of wind.

Option 3, the barometer, is an instrument used to measure atmospheric pressure, not wind speed. It is commonly used in weather forecasting.

Therefore, the anemometer is specifically designed to measure wind speed, making option 4 the correct answer.