Find the value

$=x y\left(x^{3} y^{3}-1\right)$

$=x y\left((x y)^{3}-1^{3}\right)$

$=x y(x y-1)\left((x y)^{2}+x y+1+12\right)$

$\therefore\left[x^{3}-y^{3}=(x-y)\left(x^{2}+x y+y^{2}\right)\right]$

$=x y(x y-1)\left(x^{2} y^{2}+x y+1\right)$

$\therefore x^{4} y^{4}-x y=x y(x y-1)\left(x^{2} y^{2}+x y+1\right)$