Preheat the oven to 400ºF.
Grease a 9-inch pie plate; set aside.
On a lightly floured work surface, divide pastry dough evenly in 2. Roll one out and place in the prepared pie plate.
Mix 2 tsp. flour with 2 tsp. sugar in a small bowl before sprinkling on the bottom of the crust; set aside.
In large bowl, combine blueberries, apples, blackberries and raspberries.
In another bowl, combine sugar, flour and cornstarch; toss with the fruits to coat evenly.
Transfer the mixture to the pie shell; dot with butter and sprinkle on lemon juice.
Roll out remaining pastry. Moisten the rim of pastry shell and fit pastry over filling, pressing gently; trim the edge and crimp.
Brush the milk over the top crust and sprinkle with some granulated sugar.
Transfer to the preheated oven and bake for 50-60 minutes or until the pastry is golden and the filling is bubbly.
Remove from the heat and let the pie cool on a wire rack before serving.