In a medium saucepan, melt 4 tablespoons butter over medium heat. Add 1/2 cup sugar, jelly and cranberries. Cook until cranberries are just softened, about 3 minutes. Strain cranberries over a bowl, reserving liquid. Return liquid to saucepan and simmer over medium-low heat until thickened. Set aside to cool. In prepared cake pan, arrange strained cranberries in a single layer. Pour 1/2 cup of the reduced liquid over the cranberries, discarding any remaining. Refrigerate while preparing remainder of cake.