4. Let $0<a<1$, if $x_{1}=a, x_{2}=a^{x_{1}}, x_{3}=a^{x_{1}}, \cdots, x_{n}=a^{x_{\cdots}} \cdots, \cdots$ then the sequence $\left\{x_{n}\right\}$
A. is increasing
B. is decreasing
C. odd terms are increasing, even terms are decreasing
D. even terms are increasing, odd terms are decreasing