Heat 3 cups of the pomegranate juice in a small saucepan until hot. Add the remaining cup of pomegranate juice to a large bowl and sprinkle the gelatin packets over the juice. Let sit for 1 minute. Add in the hot juice and stir until the gelatin is completely dissolved. Pour the mixture into a 9-by-13-inch pan. Sprinkle with the pomegranate seeds and grapes and refrigerate until fully set, at least 3 hours or overnight.
Put the Greek yogurt in a large bowl and whip with an electric mixer until fluffy. Cut the pomegranate gelatin into 1 1/2-inch cubes. Layer the gelatin, whipped yogurt, sliced strawberries and blueberries in 8 parfait or juice glasses.