4. Given that the area $S$ and the internal angle $A$ of $\triangle A B C$ are constants. Then the range of values for the length $a$ of side $B C$ is $\qquad$ .
4. $\because S=\frac{1}{2} b c \sin A$,
$$
\begin{array}{l}
\therefore \quad b c=\frac{2 S}{\sin A \text {. }} \\
|B C|^{2}=a^{2}=b^{2}+c^{2}-2 b c \cos A \geqslant 2 b c-2 b c \cos A \\
=4 S \cdot \frac{1-\cos A}{\sin A}=4 S \text { tan } \frac{A}{2} \text {, } \\
\therefore a \geqslant 2 \sqrt{S \text { tan } \frac{A}{2}} \text {. } \\
\end{array}
$$

When $b$ (or $c) \rightarrow+\infty$, $c$ (or $b) \rightarrow 0$,
we have, $a \rightarrow+\infty$.
Thus, $a \in\left(2 \sqrt{S \text { tan } \frac{A}{2}},+\infty\right)$.