Bevel or Bullnose a Slate Tile
Your question is more about your personal grinding skill. A grinder is pretty rough and you could expect it to break the slate and look pretty ragged, unless you are just awesome. Perhaps a dremel tool would be ok... which will probably take much longer than you want.

A better idea would be to use a diamond bladed (a concrete blade) circular saw, because it will probably not completely shatter the slate. 

If I was beveling a 4" piece, I would maybe sandwich it between to other pieces of slate with water based glue to help hold it and prevent the edges from breaking. Then soak it in water to remove the glue.