Heat the oven to 350°F. In a mixing bowl, combine the ground beef, breadcrumbs, egg, onion, 2 cloves of the minced garlic, Worcestershire sauce, ½ teaspoon of the salt, and ¼ teaspoon of the black pepper. Mix until evenly blended, then shape the mixture into a loaf and set it in a loaf pan.
Spread the ketchup evenly over the top of the loaf. Place the pan in the oven and bake for 50 minutes, or until the internal temperature reaches 160°F.
While the meatloaf bakes, place the cubed potatoes in a large pot with the remaining 2 cloves of minced garlic. Cover with water and add the remaining ½ teaspoon salt. Bring to a boil and cook until the potatoes are fork-tender, about 15 minutes.
Drain the potatoes and return them to the pot. Add the butter, milk, and remaining ¼ teaspoon black pepper. Mash until smooth and creamy.
Remove the meatloaf from the oven and let it rest for 5 minutes before slicing. Serve slices alongside the mashed potatoes.