Cook cauliflower until tender; drain. Combine ham and mushrooms. In saucepan on low heat, melt 4 tablespoons butter; add flour, then milk. Stir until thick and add cheese and sour cream. Stir until cheese melts. Combine with cauliflower and ham mixture. Turn into 2-quart casserole. Combine crumbs and remaining butter and sprinkle over top. Bake, uncovered, at 350° for 20 minutes or until hot.