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.
References
Fourth hand Wikipedia(Text) CC BY-SA