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