4. Given that $F_{1}, F_{2}$ are the two foci of the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1(a>b>0)$, and $A B$ is a chord passing through $F_{1}$, then the perimeter of $\triangle A B F_{2}$ is
A. $2 a$
B. $4 a$
C. $8 a$
D. $2 a+2 b$
4. $\mathrm{B}$ Hint: $|A B|+\left|B F_{2}\right|+\left|A F_{2}\right|=\left|B F_{1}\right|+\left|B F_{2}\right|+\left|A F_{1}\right|+\left|A F_{2}\right|=2 a+2 a=4 a$.