Let \( E \) be an ellipse whose axes are parallel to the co-ordinates axes, having its centre at \( (3,-4) \), one focus at \( (4,-4) \) and one vertex at \( (5,-4) \). If \( mx-y=4, \ m>0 \) is a tangent to the ellipse \( E \), then the value of \( 5m^{2} \) is equal to ____________.
