4. Given $0<b<1,0<a<\frac{\pi}{4}$, then the size relationship among the following three numbers: $x=(\sin a)^{\log _{s} \sin a}, y=$ $(\cos a)^{\log _{6} \cos a}, z=(\sin a)^{\log _{6} \cos a}$ is
(A) $x<z<y$;
(B) $y<z<x$;
(C) $z<x<y$;
(D) $x<y<z$.
4. (A)
$\because \quad 0\log _{b} \cos a>0, \\
\therefore & (\sin a)^{\log _{b} \sin a}z ;
\end{array}
$$

Also, $(\sin a)^{\log _{b} \cos a}<(\cos a)^{\log _{8} \cos a}$, which means $z<y$, so we have $x<z<y$,