6. As shown in Figure 2, in rectangle $A B C D$, it is known that the diagonal length is 2, and $\angle 1=\angle 2=\angle 3=\angle 4$. Then the perimeter of quadrilateral $E F G H$ is ( ).
(A) $2 \sqrt{2}$
(B) 4
(C) $4 \sqrt{2}$
(D) 6
6. B.

According to the reflection relationship, Figure 6 can be drawn. The hypotenuse of $\mathrm{Rt} \triangle I J K$ is the perimeter of quadrilateral $E F G H$.

The legs of the right triangle are twice the lengths of the rectangle's sides, and since the diagonal of the rectangle forms a right triangle with the two sides of the rectangle, the perimeter of quadrilateral $E F G H$ is twice the length of the diagonal.