Preheat oven to 400 degrees. Halve each apple lengthwise. Using a melon baller or grapefruit spoon, scoop out seeds, forming a small crater in center of each half.
Fill each crater with 1 marshmallow quarter and 1/4 of the chocolate. Cut 2 tablespoons butter into small pieces, and divide among apples, placing over chocolate.
Place water and remaining 2 tablespoons butter in a baking dish. Place filled apples in dish. Bake until a paring knife inserted into apples meets no resistance, 20 to 30 minutes.
Set oven to broil. Top each apple half with 2 marshmallow halves. Broil until golden brown. Drizzle with juices.
Cooking times for different types of apples vary; begin checking for doneness after 20 minutes.