4. Given an arithmetic sequence $\left\{a_{n}\right\}$, the sum of the first $n$ terms is $S_{n}$, and $\frac{S_{25}}{a_{23}}=5, \frac{S_{45}}{a_{33}}=25$. Then $\frac{S_{65}}{a_{43}}=(\quad$.
(A) 125
(B) 85
(C) 45
(D) 35
4. C.

Since $S_{25}=25 a_{13}$, then $\frac{a_{13}}{a_{23}}=\frac{1}{5}$.
Similarly, $\frac{a_{23}}{a_{33}}=\frac{5}{9}$.
Thus, $\frac{a_{33}}{a_{43}}=\frac{9}{13}$.
$$
\text { Therefore, } \frac{S_{65}}{a_{43}}=\frac{65 a_{33}}{a_{43}}=45 \text {. }
$$