Folded Square

In the diagram below, the line EB has been added. The dotted line must be the perpendicular bisector of EB, since for B to fold onto E, B and E must be the same distance on either side of the fold.



Using gradient
The distance CE is half of the distance BC, so the gradient of EB must be 2.
This means that the gradient of the dotted line must be $-\frac{1}{2}$.

”‹The dotted line and the folded line cross halfway between E and B (since EB is bisected), so 4 cm from the top (and from the bottom) and 2 cm from the right (and from E). So the distance marked on the diagram below is 6 cm.
 
So the vertical distance between the point where the triangle meets AD and the point where the dotted line and the folded line cross is $\frac{1}{2}\times$6 = 3 cm.

So the dotted line meets AD 4$-$3 = 1 cm below A.


Using similar triangles
The two triangles coloured blue are similar, as they both contain a right angle and their other two angles are 90$^\text{o}$ rotations of each other.

The dotted line and the folded line cross halfway between E and B (since EB is bisected), so 4 cm from the top (and from the bottom) and 2 cm from the right (and from E). So the distance marked on the diagram below is 6 cm.
 
The sides of the larger triangle are in the ratio 1:2 (from 4:8), so the vertical side of the smaller triangle must be 3 cm. So the dotted line meets AD 4$-$3 = 1 cm below A.