If a matching nut and bolt have a class 4 fit, how will they perform when being turned by hand?

When a matching nut and bolt are specified to have a class 4 fit, this denotes a medium clearance, which means that while they are designed to fit together without excessive play, there is some tightness in the fit relative to other classes.

In practical terms, this means that when attempting to turn the bolt with just your hand, it will require a noticeable amount of force, making it challenging to turn. The clearance is not so tight that it causes the bolt and nut to jam, yet it does create enough resistance that hand twisting becomes difficult. However, once a wrench is applied, the mechanical advantage provided by the tool allows for much easier turning, as the additional leverage compensates for the friction and resistance created from the close fit.

This understanding of the fit characteristics confirms why the chosen answer is appropriate. A class 4 fit is engineered for a reliable connection while still allowing for effective performance when using tools, which is why using a wrench would facilitate easy turning that would not be achievable by hand alone.