The diagonal of the square is the radius of the circle. To find the diagonal, you can draw a line between 2 opposite points of the square to split the square into 2 equal triangles. As they are right triangles and have 2 equal sides, they are 1-1-√2 triangles, or 45-45-90 (degrees). The length of the 2 equal sides of the triangle is 4 inches, to multiplying that by √2 will give 4√2 inches, which is the radius of the circle.