Preheat grill to medium-high (400-450 degrees F). Place 4 (12-inch) foil squares in a single layer on a work surface; coat with cooking spray. Place 1 salmon fillet, skin-side down, in the center of each. Top each fillet with 1 1/2 teaspoons butter, 1/4 teaspoon lemon pepper, 1/8 teaspoon salt, 2 lemon slices and 1 parsley sprig. Crimp the sides of each foil packet together to tightly seal. Place the packets on the grill; cover and grill until the fish flakes easily with a fork, 8 to 10 minutes.