In a small bowl, combine the honey, butter, horseradish and mustard. Pour 1/3 cup marinade into a large resealable plastic bag; add the tenderloin. Seal bag and turn to coat; refrigerate for 6-8 hours, turning occasionally. Cover and refrigerate remaining marinade for basting.
Drain and discard marinade from pork. In a large skillet, brown pork on all sides. Place on a rack in a foil-lined shallow roasting pan. Bake, uncovered, at 375° for 35-40 minutes or until a meat thermometer reads 160°, basting occasionally with reserved marinade. Let stand for 5 minutes before slicing. Yield: 2 servings.