Explanation:

- Option 1: A positive force implies attractive nature.

This is incorrect. In most physics contexts (like electrostatics or gravity), a positive sign often indicates repulsion.

- Option 2: A negative force implies repulsive nature.

This is incorrect. Negative forces generally point towards attraction.

- Option 3: A positive force can be both attractive and repulsive in nature.

This is incorrect in typical force direction conventions. The sign defines the type.

- Option 4: A negative force implies attractive nature.

This is the correct answer. For example, in gravitational and electrostatic forces, a negative sign usually shows the force is attractive in nature.