\[xy^{4} - y^{4} + xy^{3} - y^{3} =\]
\[= y^{4}(x - 1) + y^{3}(x - 1) =\]
\[= (x - 1)\left( y^{4} + y^{3} \right) =\]
\[= y^{3}(y + 1)(x - 1).\]