ABCD is a rectangle whose three vertices are B(4, 0), C(4, 3) and D(0, 3).

The given vertices are B(4, 0), C(4, 3) and D(0, 3). Here, BD is one of the diagonals. So

$B D=\sqrt{(4-0)^{2}+(0-3)^{2}}$

$=\sqrt{(4)^{2}+(-3)^{2}}$

$=\sqrt{16+9}$

$=\sqrt{25}$

$=5$

Hence, the length of the diagonal is 5 units..