A uniform square lamina \(A B C D\) of side \(4 a\) and weight \(W\) rests in a vertical plane with the edge \(A B\) inclined at an angle \(\theta\) to the horizontal, where \(\tan \theta=\frac{1}{3}\). The vertex \(B\) is in contact with a rough horizontal surface for which the coefficient of friction is \(\mu\). The lamina is supported by a smooth peg at the point \(E\) on \(A B\), where \(B E=3 a\) (see diagram).
(i) Find expressions in terms of \(W\) for the normal reaction forces at \(E\) and \(B\).

(ii) Given that the lamina is about to slip, find the value of \(\mu\).