1. Preheat oven to 450°.
2. Remove and discard giblets and neck from turkey. Rinse turkey with cold running water, and pat dry. Trim excess fat. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, gently pushing between skin and meat. Combine mustard, sage, black pepper, and garlic. Rub mustard mixture under loosened skin over flesh. Lift wing tips up and over back; tuck under turkey. Secure legs with kitchen twine.
3. Place turkey, breast side up, in a shallow roasting pan. Pour 2 cups broth over turkey. Place turkey in oven; reduce oven temperature to 325°.
4. Bake, uncovered, at 325° for 2 hours and 20 minutes or until a thermometer inserted in thickest part of thigh registers 165°, basting turkey frequently with pan juices. Transfer turkey to a large serving platter, reserving 1/4 cup drippings. Loosely cover turkey with foil, and let stand 25 minutes.
5. Combine reserved drippings, flour, and next 4 ingredients in a medium saucepan, stirring with a whisk until smooth. Gradually stir in remaining 2 cups broth. Bring to a boil over medium-high heat, stirring constantly with a whisk. Reduce heat, and simmer 2 minutes or until gravy thickens, stirring occasionally. Remove from heat.
6. Discard turkey skin; carve. Serve turkey with gravy.