4. Let the function $f(x)$ have the domain $D=(-\infty, 0) \bigcup(0,+\infty)$, and for any $x \in D$, it holds that $f(x)=\frac{f(1) \cdot x^{2}+f(2) \cdot x-1}{x}$, then the sum of all zeros of $f(x)$ is $\qquad$ .