$4^2+x^2={\left(4\sqrt{5}\right)}^2$  

$16+x^2=4^2\cdot {\left(\sqrt{5}\right)}^2$  

$16+x^2=16\cdot 5$  

$16+x^2=80\mid -16$   

$x^2=64\mid \sqrt{}$  

$x=8$  

$\text{TAK, poniewaz˙ B.}$