A fourth hand is the name of a specialized bicycle tool for manipulating brake and derailleur cables. The fourth hand allows a mechanic to keep the cable in place while they adjust the tension. Generally the fourth hand contains a locking mechanism. The fourth hand is sometimes called a "cable puller" or "cable stretcher." Modern fourth hands are an improvement on an earlier generation of cable stretchers known as "third hands," hence the name.
See also Bicycle mechanic