Preheat oven to 375°F (190°C). Line a rimmed baking sheet with parchment paper.

If needed, hard-boil the four filling eggs, cool them completely in ice water, peel, and pat dry.

In a large bowl, combine breadcrumbs and milk. Let stand for 5 minutes so the crumbs absorb the liquid.

Add ground beef, beaten egg, onion, garlic, parsley, Worcestershire sauce, Dijon mustard, smoked paprika, salt, and pepper. Mix gently just until evenly combined.

Place about two-thirds of the meat mixture on the prepared baking sheet and shape it into a 10-by-6-inch rectangle with a shallow trench down the center.

Arrange the four peeled hard-boiled eggs end to end in the trench. Cover with the remaining meat mixture, pressing the edges together to fully seal the eggs inside. Shape into an even loaf.

In a small bowl, stir together ketchup, brown sugar, Dijon mustard, and apple cider vinegar. Spread half of the glaze over the meatloaf.

Bake for 45 minutes. Spread the remaining glaze over the top and continue baking for 15 to 20 minutes, or until the center of the meat reaches 160°F (71°C).

Rest the meatloaf for 10 to 15 minutes before slicing. Cut with a sharp knife so each serving reveals a neat cross-section of egg.