Preheat oven to 400 degrees. Melt half the butter in a small saucepan.
Put 2 tablespoons melted butter in a roasting pan and arrange squash slices in one layer on it. Sprinkle with salt and pepper and drizzle remaining 2 tablespoons melted butter on top. Roast without turning for 20 to 30 minutes, or until squash is tender; keep warm while you finish fish.
About 5 minutes before squash is done, put a heavy skillet over medium-high heat for about 2 minutes; add 2 tablespoons butter. Dredge cod pieces lightly in flour and sprinkle with salt and pepper. Add to skillet and turn heat to high; cook, turning only once, until nicely browned on both sides and cooked through (a thin-bladed knife will meet little or no resistance when fish is done).
Arrange squash slices on a platter, then top with fish. Add remaining butter to skillet over medium heat; when it sizzles and browns, add vinegar and cook for 10 to 20 seconds more. Pour over all and serve immediately.