Thaw the 4 puff pastry sheets at room temperature. Wash the apples, remove the cores, cut into quarters, and cut the flesh into small cubes.
Place the apple cubes in a saucepan with the lemon juice and 2 tablespoons of sugar. Heat over low heat and cook for 5 minutes. Remove from heat and let cool for 2-3 minutes.
Stir the quark until smooth and creamy, then beat in the remaining 2 tablespoons of sugar. Sprinkle the chopped almonds over each puff pastry sheet. Preheat the oven to 200°C (fan) / 220°C (conventional) / 425°F. Line a baking sheet with parchment paper.
Place 1/4 of the quark mixture in the center of each pastry sheet and spread gently, leaving a 1-2 cm margin around the edges. Top with the cooled apple mixture. Brush the pastry edges with water. Fold each sheet into a triangle and press the edges together firmly with a fork.
Place the triangles on the prepared baking sheet. Brush with milk. Bake in the preheated oven for 15 minutes.