\[= - 4 \cdot (a - 4)^{2}\]
\[- 4 \cdot (a - 4)^{2} < 0\ \ \]
\[(a - 4)^{2} \geq 0\ \ \]
\[a \neq 4\]
\[Ответ:a \in ( - \infty;4) \cup (4;\ + \infty).\]