Vertices of surface

I am trying to get vertices of surface, but only corners and not the control points. Deconstruct gives all the points which include control points. I do understand it has to do with how the surface is made butt hats the information have and cannot control it. What I want to go ahead with is getting just the 4 vertices.

I have been struggling to get this . Any help is appreciated. 

Attached is the snippet of file.