shrinking an edge along the xy axis and to a 90 degree angle

If I take a cube and bevel an edge to 4 segments and then select the middle edge and want to shrink it so the two connecting faces form a 90 degree angle and so the z position is constrained. What keys do I use to do this? Thanks

Using the transform menu I had to move the 2 selected vertices to 1 and -1 position respectively. As well I had to adjust the shrinkage so I have about 90 degrees between the 2 adjacent faces.