Hmm, not sure what year they started fitting them, but you might find that the four holes are actually blocked at the bottom with a tiny grub screw in each hole.
Get a torch, shine it into the holes and have a look - If this is the case you'll need an allen key of suitable size to remove the grub screws, before using the standard 2 x 2-prong keys to get the head unit out.
If the H/U isnlt fitted with the grub screws, it should just come straight out with the 2 pronged tools (or a wire coat hanger if you;re stuck for proper tools!)