well, 9000 - 4000 = 5000. If we take 9000 to be the 100%, how much in percent is 5000 off of it?
[tex]\begin{array}{ccll} amount&\%\\ \cline{1-2} 9000&100\\ 5000&x \end{array}\implies \cfrac{9000}{5000}=\cfrac{100}{x}\implies \cfrac{9}{5}=\cfrac{100}{x} \\\\\\ 9x=500\implies x=\cfrac{500}{9}\implies x=55.\overline{555}\implies \stackrel{\textit{rounded up}}{x\approx 55.6}[/tex]