I think you're only supposed to get 2 and you're supposed to use the screws that come with the mount itself. Although in my case with the Ergotron MX Desk mount I had the problem that the screws where too long. Had to use a nut and two spacers to fill up the empty space.
Photo of my 'solution' (it's a 5megapixel photo so I'm not going to use the image tag).
For photo's of a non-glass (it's a WD instead of WC but who cares):
Photo 1
Photo 2
Do note that I was going for a worst possible situation with the sun shining through the window. Is it refective? Yes. Is it better than the alternative (grainy AG-coating)? In my opinion, yes!