Draw a circle with radius 4 cm and center P. Draw another circle of same radius and with center at Q such that it intersects the previous circle. Join PQ. Now join the points of intersection and name this line segment as AB. Mark the point of intersection of AB and PQ as O.
If you draw step by step as given in the question, you get:
In the above figure
Line AB is perpendicular to Line PQ and both the lines intersect each other at O.
∴ ∠AOP = ∠AOQ = 90∘