You may have made a mistake with your signs while using FOIL, obtaining:

$\displaystyle{\sqrt[{{4}}]{{{{x}}^{{4}}}}}+{\sqrt[{{4}}]{{{{x}}^{{3}}}}}+{\sqrt[{{4}}]{{{{x}}^{{3}}}}}-{\sqrt[{{4}}]{{{{x}}^{{2}}}}}\usepackage{cmbright}\Huge\$