velocity of car at $t=4\sec$ is

$v=u+at$

$v=0+5\left(4\right)$

$=20 \mathrm{m} {\mathrm{s}}^{-1}$

At $t=6\sec$

acceleration is due to gravity $\therefore a=g=10 \mathrm{m} {\mathrm{s}}^{-2}$

$v_{\mathrm{x}}=20 \mathrm{m} {\mathrm{s}}^{-1}$ (due to car)

$v_y=u+at$ 

$=0+g\left(2\right)$ (downward)

$=20 \mathrm{m} {\mathrm{s}}^{-1}$ (downward)

$v=\sqrt{v_x^2+v_y^2}$

$=\sqrt{{20}^2+{20}^2}$

$=20\sqrt{2} \mathrm{m} {\mathrm{s}}^{-1}$