The centre of a square of side 4 units length is \( (3, 7) \), and one of the diagonals is parallel to the line \( y = x \).
If \( (x_1, y_1), (x_2, y_2), (x_3, y_3), (x_4, y_4) \) are the vertices of this square, then
\[
\frac{y_1 y_2 y_3 y_4}{x_1 x_2 x_3 x_4} = ?
\]