Prepare the foil packets: Preheat your grill to medium heat. Tear off four large sheets of aluminum foil (about 12 inches long each). Set aside.
Prepare the ingredients: In a large bowl, toss the shrimp, corn, potatoes, and sausage with melted butter, Old Bay seasoning, garlic powder, paprika, and black pepper. Make sure everything is evenly coated with the seasoning mix.
Assemble the packets: Divide the mixture evenly between the four sheets of foil. Place a couple of lemon slices on top of each packet for added flavor. Fold the foil over the ingredients to form a secure packet, making sure the edges are tightly sealed.
Grill the packets: Place the foil packets on the preheated grill. Grill for 15-20 minutes, or until the shrimp are cooked through, the corn is tender, and the potatoes are soft. You can check by carefully opening one packet to see if the potatoes are fully cooked.
Serve: Carefully open the packets (steam may escape, so be cautious). Garnish with freshly chopped parsley and serve immediately.