\[y^{4} - y^{3} + y - 1 =\]
\[= y^{3}(y - 1) + (y - 1) =\]
\[= \left( y^{3} + 1 \right)(y - 1) =\]
\[= (y + 1)\left( y^{2} - y + 1 \right)(y - 1).\]