$${ x }^{ 4 } - { \left(x-z \right) }^{ 4 }$$
$z\left(2x-z\right)\left(\left(x-z\right)^{2}+x^{2}\right)$
$4xz^{3}-z^{4}+4zx^{3}-6\left(xz\right)^{2}$
