2. Given the equation in $x$, $x^{4}+k x^{2}+4=0$, has four distinct real roots. Then the range of values for $k$ is $\qquad$ .
$$
\left\{
\begin{array}{l}
k0, \\
y_{1}+y_{2}=-k>0, \\
y_{1} y_{2}=4>0 .
\end{array}\right.
$$

Solving yields $k<-4$.