Add all four ingredients to a saucepan, and place over a medium heat. Stir thoroughly until the mixture melts together. Bring to a simmer, and cook for a couple more minutes, stirring constantly.
Transfer the chocolate mixture to a lined, greased dish. When cooled, place in the fridge, and leave to set overnight.
When fully set, turn the fudge out from the dish, and cut into chunks. You may find it easier to cut if you dip the knife in hot water first, since it's quite sticky. Store the fudge in the fridge.