4. The graph of the quadratic function $y=$ $a x^{2}+b x+c$ is shown in Figure 3. Also, $k=$ $\frac{b(2 a+b)(a+b+c)}{c(2 a-b)(a-b+c)}$, then the sign of $k$ is $\qquad$
$4 . k>0$.
From Figure 3, we can see that the intersection point with the $y$-axis $(0, c)$ is below the $x$-axis, so $c<0$;
When $x=-1$, $y=a-b+c<0$.