Four point charges are fixed in the \(xy\)-plane on the coordinate axes at a distance \(a\) from the origin, as shown in the figure. Two positive charges \(+q\) are located at \((a,0)\) and \((-a,0)\), and two negative charges \(-q\) are located at \((0,a)\) and \((0,-a)\). A point \(P\) is located on the positive \(x\)-axis at position \(x = x_0\), where \(x_0 \gg a\). Assuming electric potential is zero at infinity, which of the following expressions best approximates the electric potential \(V\) at point \(P\)?