A chiro pointed out there are two different regions bounded by that circle and line- the part of the disk above the line and the part below the line. But in neither is the integral of a single function from 0 to 4. If you are finding the area of the part of the disk above the line, your integral stops where the line intersects the circle. If you are finding the area of the part of the disk below the line, you will have one integrand before the x value where the line intersects the circle and another after.