4. If the focal distance, minor axis length, and major axis length of an ellipse form an arithmetic sequence, then its eccentricity is ( ).
A. $\frac{3}{4}$
B. $\frac{\sqrt{2}}{3}$
C. $\frac{3}{5}$
D. $\frac{9}{10}$
4. C

Let the focal distance of the ellipse be $2 c$, the length of the minor axis be $2 b$, and the length of the major axis be $2 a$,
then $a^{2}=b^{2}+c^{2}$
$$
a+c=2 b
$$

Substituting (2) into (1) gives $4 a^{2}=(a+c)^{2}+4 c^{2}$
Thus, $(3 a-5 c)(a+c)=0$. Therefore, $a=\frac{5}{3} c$.
Substituting into (2) gives $b=\frac{4}{3} c$
Therefore, $e=\frac{c}{a}=\frac{3}{5}$.