A circumscribed angle is that which is formed by the intersection of the two tangent lines in a circle. With this, we can conclude that segments AC and AB are tangent to circle O. The tangent lines forms a right angle with the radius of the circle drawn from the center of the circle to the tangent point.
By the explanation above, we can say that angles C and B are equal to 90° and that triangle ACO and triangle ABO are congruent. Which means that segment AC is equal to segment AB. Thus, the length of AB is also 4.
