Melt the remaining 4 tablespoons of butter in the skillet. Add the radishes, shallots and crisp bacon and cook over moderately high heat, stirring a few times, until the radishes are golden brown, about 6 minutes. Add the sugar and cook for 2 minutes, until dissolved. Add the orange juice and boil, stirring a few times, until the radishes are barely tender and the sauce is lightly thickened, about 2 minutes. Stir in the radish greens, season with salt and pepper and serve.