Given a circle is passing through all the vertices of the rectangle ABCD.
Since, ∠BAD=∠BCD=90∘,
BAD and BCD are semicircles as angle in a semi-circle is 90∘.
So, BD is the diameter of the circle which is also the diagonal of the rectange.
Diagonal, BD = 2 x (radius) = 2 x 4 = 8 cm