Cook potatoes according to package directions, omitting salt. Stir in 1 1/2 teaspoons salt.
Melt 4 tablespoons butter in a small skillet over high heat; add garlic, and sauté 2 minutes. Stir garlic and sour cream into mashed potatoes.
Transfer potato mixture to a lightly greased 13- x 9-inch baking dish. Dot with 3 tablespoons butter. (Cool completely, cover, and refrigerate up to 24 hours, if desired.)
Bake at 350° for 25 minutes. Top evenly with cheese, if desired, and bake 5 more minutes or until melted.