4. (2007 Jiangxi Province College Entrance Examination Question) Let the function $f(x)$ be a differentiable even function on $\mathbf{R}$ with a period of 5, then the slope of the tangent line to the curve $y=f(x)$ at $x=5$ is
A. $-\frac{1}{5}$
B. 0
C. $\frac{1}{5}$
D. 5