Preheat oven to 275 degrees F (135 degrees C). Butter a 9x13-inch baking pan.

Mix 4 cups and 2 tablespoons flour, butter, and white sugar together in a bowl using your hands until completely mixed. Spread dough into the prepared baking pan; cover with waxed paper and press firmly into pan. Prick dough through the waxed paper using a fork; remove waxed paper.

Bake in the preheated oven until shortbread is lightly browned, about 1 1/2 hours. Turn off oven and remove pan. Cut shortbread into 1-inch squares. Return pan to oven until oven is completely cooled.

Sprinkle shortbread with colored sugar, remove squares from pan, and store in an airtight container.