Integrating a circle with rectangular elements

Imagine a circle where its center is at the origin with radius r. Now suppose I make infinitesimally thin rectangles only at the first quadrant, the rectangles length>width, from that one can see that I'm just trying to find a fourth of the total area of that circle. In terms of why, I have set up my integration problem to be: the integral from 0 to 4 of sqrt(r^2-x^2)dx times 4(since I'm trying to find the whole area, not just a fourth of the area). From that I'm just obviously trying to make the answer end up being (pi)r^2, the equation of a circle, but I can't figure out how to evaluate the integral from 0 to 4 of sqrt(r^2-x^2)dx times 4, unless its wrong... Anyways if it were right, how would I even do the integral, please show me your work, thank you.