Final answer:
The unique action that the four muscles in the arm perform is supination, which involves the outward rotation of the forearm showing the palm upward or forward.
Step-by-step explanation:
The action that the four muscles of the arm perform that no other muscles studied perform is supination. Supination of the forearm refers to the outward rotational movement of the forearm that causes the palm to face upward or forward when in the anatomical position. This action is in contrast with pronation, which is the inward rotation of the forearm, causing the palm to face downward or backward. Muscles like the biceps brachii are involved in this motion. It is important to note that muscles can only contract and therefore must work in opposing pairs to perform actions. For instance, while the biceps brachii assists in supination, the triceps brachii acts as an antagonist during this movement to control and coordinate the action.