(i) To find the equation of AC, use the points A(0, 4) and C(8, 0). The gradient \(m\) is given by \(m = \frac{0 - 4}{8 - 0} = -\frac{1}{2}\). The equation of the line is \(y = mx + c\). Substituting the point A(0, 4), we get \(4 = -\frac{1}{2}(0) + c\), so \(c = 4\). Therefore, the equation of AC is \(y = -\frac{1}{2}x + 4\).

For OB, since AC is the line of symmetry, OB is perpendicular to AC. The gradient of OB is the negative reciprocal of \(-\frac{1}{2}\), which is 2. Therefore, the equation of OB is \(y = 2x\).

(ii) To find the coordinates of B, use the midpoint formula for OB. Since O is the origin (0, 0), the midpoint of OB is \(\left( \frac{0 + x}{2}, \frac{0 + y}{2} \right)\). Given that the midpoint is on AC, substitute into the equation \(y = -\frac{1}{2}x + 4\). Solving the simultaneous equations \(y = 2x\) and \(y = -\frac{1}{2}x + 4\), we find \(x = 3.2\) and \(y = 6.4\). Therefore, the coordinates of B are (3.2, 6.4).