The standard equation of a circle with center at the origin is x² + y² = r², where r is the radius. Substituting 4 for x and 0 for y yields 4² + 0² = r², so we see immediately that r = 4 units.
The formula for the area of a circle is A = πr².
Here, with r = 4, the area is A = π(4 units)² = 16π units², or
