Based on my poor memory of doing this repair, I'll try to help.
Look at the pictures in this ebay listing:
ebay listing velvac 709645If you look at the second picture in that listing, you'll see the back side of the backing plate. The circle part of that backing plate will snap into a circular mount for the motorized function of mirror adjustment. (It snaps into what wa8yxm above called the "trolley".) The four screws are in the part that the backing plate snaps into.
From your description, I do not think you are replacing anything but the mirror and the backing plate and will not need to remove those screws.
Fred
