If the radius of a circle is 4, what is the diameter?

The diameter of a circle is directly related to the radius by a simple mathematical formula. Specifically, the diameter is always twice the length of the radius. In this case, if the radius is 4, the calculation for the diameter would be:

Diameter = 2 × Radius

Diameter = 2 × 4

Diameter = 8

This means that the correct answer, which states that the diameter is 8, accurately reflects the relationship between the radius and diameter of a circle. Understanding this relationship is fundamental in geometry and is applicable in various problems involving circles.