Tool to help precision drill 4 holes in a wall?
I make a template with a piece of paper or cardboard (like the box the item came in).  This makes drilling holes in a precise pattern a bit easier.  In some situations, you might transfer the pattern from your paper onto the wall using pencil / marker / spray-paint.
For the size mirror you're talking about, you might want to read about another mounting option, called a French cleat.  It involves attaching a piece of wood to the wall, and another to the item being hung up.  The two pieces fit together with a slant.  This not only makes it easier to hang the item, but also, to remove it later if you need to paint or otherwise access the wall behind it.