Preheat the pellet smoker to 250°F. Fill the hopper with applewood pellets and place a small rimmed baking sheet or foil-lined pan on the counter for shaping the meatloaf.
Finely chop 2 slices of bacon. In a large bowl, combine the chopped bacon, ground beef, breadcrumbs, egg, milk, onion, Worcestershire sauce, salt, black pepper, and garlic powder. Mix gently just until combined to help keep the meatloaf tender.
Shape the mixture into a compact loaf on the baking sheet. Lay the remaining 2 bacon slices across the top, tucking the ends underneath if needed.
Set the pan in the smoker and cook until the center of the meatloaf reaches 150°F, about 1 1/2 hours. Keep the lid closed as much as possible so the smoker maintains a steady temperature.
While the meatloaf smokes, stir together 1/4 cup of the ketchup, apple cider vinegar, and brown sugar in a small bowl. Brush the mixture over the meatloaf during the final 20 to 30 minutes of cooking.
Continue cooking until the center reaches 160°F. Remove the pan from the smoker and let the meatloaf rest for 10 minutes before slicing. Serve with the remaining 1/4 cup ketchup on the side.