\[\left( 4 - y + y^{2} - y^{5} \right)(1 - y) =\]
\[= 4 - 4y - y + y^{2} + y^{2} - y^{3} - y^{5} + y^{6} =\]
\[= y^{6} - y^{5} - y^{3} + 2y^{2} - 5y + 4.\]