\[y^{4} - 8y^{2} + 16 =\]
\[= \left( y^{2} \right)^{2} - 8y^{2} + 16 =\]
\[= \left( y^{2} - 4 \right)^{2} = (y - 2)^{2}(y + 2)^{2}.\]