Heat the oven to 350°F. In a small bowl, mix the ketchup, soy sauce, mustard, and ginger. Taste and adjust the seasoning. Keep the sauce at room temperature while you make the katsu.
Put the panko in a dish. Shape the ground pork into four 1/2-inch-thick patties and sprinkle them generously with salt and pepper. Press each patty into the panko to coat and flatten more.
Put about 1/8 inch oil in a large skillet over medium-high heat. When it’s hot, add the patties and cook until the panko are golden, 2 to 3 minutes; turn and brown the other side. Transfer the skillet to the oven and roast until the pork is cooked through, 8 to 10 minutes. Drizzle each patty with some sauce and pass the rest at the table.