In a large saucepan, melt 3 tablespoons of the butter. Add the onions, season with salt and pepper and cook over moderate heat, stirring occasionally, until golden and tender, about 10 minutes. Add the garlic, paprika and cayenne and cook, stirring, until dark red in color, 2 minutes. Add the flour and the remaining 1 tablespoon of butter and cook, stirring, for 2 minutes. Stir in the broth and sugar and cook until the sauce is thickened, about 2 minutes. Add the chicken to the sauce and bring to a simmer. Cook over moderately low heat until the chicken is tender, about 15 minutes. Stir in the heavy cream and season with salt and pepper.