Once you have unscrewed the four screws, take your spudger/plastic opening tool and wedge it in the back of the phone as shown in the picture.

Find a very thin crevice which seems to run around the phone itself and assertively wedge the spudger into the crevice.

Once you have successfully inserted the spudger follow through by sliding the spudger assertively around the phone frame.

Apply moderate force while trying to wedge the tool in.