4. The curve $y \cos x+2 y-1=0$ is first translated $\frac{\pi}{2}$ units to the right along the $x$-axis, and then 1 unit downward along the $y$-axis. The resulting curve equation is
A. $(1-y) \sin x+2 y-3=0$
B. $(y-1) \sin x+2 y-3=0$
C. $(y+1) \sin x+2 y+1=0$
D. $-(y+1) \sin x+2 y+1=0$
4. C The original expression is transformed into $y=\frac{1}{2+\cos x}$ and then translated.