If the function
\[
f(x) =
\begin{cases}
\frac{2}{x} \left( \sin(k_1 + 1)x + \sin(k_2 -1)x \right), & x<0 \\
4, & x = 0 \\
\frac{2}{x} \log_e \left( \frac{2 + k_1 x}{2 + k_2 x} \right), & x>0
\end{cases}
\]
is continuous at \( x = 0 \), then \( k_1^2 + k_2^2 \) is equal to: