(4) If the set $A=\left\{x \left\lvert\, x=\frac{k \pi}{2}+\frac{\pi}{4}\right., k \in \mathbf{Z}\right\}, B=\left\{x \left\lvert\, x=\frac{k \pi}{4}+\frac{\pi}{2}\right., k \in \mathbf{Z}\right\}$, then the relationship between $A$ and $B$ is
(4) $A \varsubsetneqq B$