4. Given $0<b<1,0<a<\frac{\pi}{4}$, then the size relationship of the following three numbers: $x=$ $\sin a^{\log _{b} b^{\sin a}}, y=(\cos a)^{\log _{6} b^{\cos a}}, z=(\sin a)^{\log _{b} \cos a}$ is
(A) $x<z<y$;
(B) $y<z<x$;
(C) $z<x<y$;
(D) $x<y<z$.
4. (A)
$\because 0\log _{b} \cos a>0$,
$\therefore \quad(\sin a)^{\log _{b} \sin a}z$;
Also, $(\sin a)^{\log _{6} \cos \alpha}<(\cos a)^{\log \cos \alpha}$, which means $z<y$, so we have $x<z<y$,