Return pot to burner. Remove the bowl used for the milk chocolate. Bring water to a boil, then reduce heat to simmer.
Place 4 oz white chocolate in the second medium bowl and place on pot. Keep on low heat. The chocolate will start to melt. Stir frequently, until the chocolate is completely melted. Turn off heat. Allow the chocolate to cool slightly, a few minutes.
Prepare your piping bag with a tip, with a small opening. Or, you may use a Ziplock bag, cutting a small hole in the corner after you have poured in the chocolate.
Pour the chocolate into the piping bag (use your finger to cover the opening of the tip). Or, pour into a Ziplock bag, seal, and then invert and cut a small hole into one of the bottom corners.
Remove apples from the refrigerator. Leave on the sheet. Quickly make zig-zags across the tops of the apples with the chocolate. Repeat in the opposite direction. Return apples to the refrigerator to completely firm up.