Meanwhile, in a large saucepan, melt 4 tablespoons of the butter. Add the flour and cook over moderate heat, stirring, until smooth, about 1 minute. Whisk in the reserved porcini liquid and the milk. Bring to a simmer and cook over moderately low heat until thickened, about 5 minutes. Stir in the cream and chopped porcini mushrooms. Scrape into a large bowl.