Bring a pot of water to a boil, with the 4 plum tomatoes. You will need to boil the tomatoes until you notice the skin starting to peel off of the tomato. Once you see this, you can drain the water, and run the tomatoes under cold water. Let them sit for a little while, so they are not too hot when peeling off the skin. Once they cool down, peel off the skin. After peeling, cut the tomatoes into cubes.