4. Given $0<b<1,0<a<\frac{\pi}{4}$, then the size relationship of the following three numbers: $x=(\sin a)^{\log _{b} \sin a}, y=(\cos a)^{\log _{b} \cos a}, z=$ $(\sin a)^{\log _{b} \cos a}$ is
(A) $x<z<y$
(B) $y<z<x$
(C) $z<x<y$
(D) $x<y<z$
(A)
4.【Analysis and Solution】 $\because 0\log _{b} \cos a>0, \\
\therefore(\sin a)^{\log _{b} \sin a}z ; \\
\text { Also }(\sin a)^{\log _{b} \cos a}<(\cos a)^{\log _{b} \cos a}, \text { i.e., } z<y, \text { thus } x<z<y .
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