If the normal at the four points (x1, y1), (x2, y2), (x3, y3) and (x4, y4) on the ellipse \(\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1\) are concurrent, then the value of (x1 + x2 + x3 + x4) \((\frac1{x_1} + \frac 1{x_2} + \frac1{x_3} + \frac1{x_4})\) must be ____.
