I started with the 5x5 at 0 degrees (overlapping the 4x4 just in the lower left quadrant). Area of overlap = i/4 of the 4x4. Then rotate 45 degrees so the 5x5 sides hit the corners of the 4x4. Area of overlap by observation is again 1/4 of the 4x4. Since these are equal I assumed the overlap is independent of rotation.