The line \( 2x - y + 1 = 0 \) is a tangent to the circle at the point \( (2, 5) \) and the centre of the circle lies on \( x - 2y = 4 \). Then, the radius of the circle is:
\( 5\sqrt{4} \)
\( 3\sqrt{5} \)
\( 5\sqrt{3} \)
\( 4\sqrt{5} \)