What action is associated with cranial nerve 4 (trochlear nerve)?

The trochlear nerve, also known as cranial nerve IV, is primarily responsible for controlling the superior oblique muscle of the eye. This muscle is instrumental in managing the movement of the eye, particularly in directing it downward and inward. When the trochlear nerve stimulates the superior oblique muscle, it allows for controlled motion which is essential for tasks such as reading or looking down at an object that is close.

The actions associated with the movement of the eye that this nerve controls are crucial for proper function, including coordinated eye movements and maintaining binocular vision. Thus, the connection between cranial nerve IV and downward and inward eye movement is the correct understanding of this nerve's function.