讲解一下这道题---**Textual Information:** 如图, AD是⊙O的直径, △ABC是⊙O的内接三角形. 若∠DAC = ∠ABC, AC = 4, 则⊙O的直径AD = _______. **Mathematical Formulas/Equations:** * ∠DAC = ∠ABC * AC = 4 **Chart/Diagram Description:** * **Type:** Geometric figure (circle with an inscribed triangle and a diameter). * **Main Elements:** * **Circle:** A circle with center O. * **Points:** Points A, B, C, D are on the circumference of the circle. Point O is at the center of the circle. * **Lines:** * Line segment AD is a diameter of the circle, passing through O. * Triangle ABC is inscribed in the circle, formed by line segments AB, BC, and AC. * Line segment BD is drawn. * **Labels:** The points are labeled A, B, C, D, and O. * **Relative Position:** O is the center. A, B, C, D are on the circle. AD is a diameter. Triangle ABC is inscribed in the circle.