Place 1 cup grape juice in a medium bowl. Sprinkle the gelatin over the juice, and let stand for a few minutes.

Meanwhile, in a small saucepan heat the remaining 3 cups grape juice until just boiling. Remove from heat, and pour over the juice-and-gelatin mixture. Stir until the gelatin is dissolved.

Pour into a 9-inch square baking pan, and transfer pan to the refrigerator until firm, about 3 hours, or overnight.

Dip the pan briefly in hot water to loosen, cut into squares, and serve.