Find the entries $x_i$ in the matrix P so that P is an orthogonal matrix where P=

$$\begin{bmatrix}

1 & 1 & 1 & 1 \\

1 & 1 & -1 & -1 \\

1 & -1 & 1 & -1 \\

x_1 & x_2 & x_3 & x_4

\end{bmatrix}$$