4. Let $Z$ be a complex number, $A, B, C$ be real numbers, $A<0$, and let $f(Z)=A|Z|^{2}+2 B \cdot \operatorname{Re} Z+C$ (where $\operatorname{Re} Z$ denotes the real part of $Z$), then when $Z=$ $\qquad$, $f(Z)$ has the most $\qquad$ value $\qquad$ .
4. $-\frac{B}{A}, large, \frac{A C-B^{2}}{A}$