Preheat oven to 375 degrees F. Cook the cauliflower in a large pot of boiling, salted water for 4 minutes. Drain and place the cauliflower in a 2-quart buttered oven dish, sprinkle with lemon juice, then with salt and pepper. Spread the white sauce evenly over the top. Mix the bread crumbs and the 4 tablespoons of melted butter; stir in the grated cheese. Distribute the crumb mixture evenly over the top. Bake for 20 minutes.