Stir 4 cups milk, sugar, and rice together in a slow cooker until sugar dissolves. Add margarine, cover, and cook on High for 2 1/2 hours.

Whisk egg, 2 tablespoons milk, vanilla, almond extract, and salt together in a bowl.

Pour a small amount hot rice mixture into egg mixture, whisking quickly so egg doesn't cook. Pour mixture back into the slow cooker and whisk until combined. Continue to cook on High until creamy, about 15 more minutes.

Transfer pudding to a glass baking dish. Cover with plastic wrap leaving two corners uncovered to vent, and refrigerate until chilled completely, at least 1 hour.