4. Let the function $f(x)=3 \sin x+2 \cos x+1$. If real numbers $a, b, c$ make $a f(x)+b f(x-c)=1$ hold for any real number $x$, then the value of $\frac{b \cos c}{a}$ is
A. $-\frac{1}{2}$
B. $\frac{1}{2}$
C. -1
D. 1