In a large, deep skillet, melt 1 tablespoon of the butter. Add the leek and cook over moderate heat, stirring occasionally, until softened, about 4 minutes. Stir in the shrimp, scallops and crab and season with salt and pepper. Add the wine and simmer just until the shrimp are pink, about 2 minutes. Using a slotted spoon, transfer the seafood to a bowl. Add the parsley and the remaining 3 tablespoons of butter to the skillet and simmer until the liquid is reduced to a few tablespoons, about 3 minutes. Return the seafood to the pan and toss to coat.