Preheat oven to 350 degrees F.
Melt butter or margarine in a large skillet over medium high heat. Add chicken breasts and saute until just cooked; Place chicken on a lined cookie sheet.
Crumble 4 of the bacon slices; Mix the crumbled bacon with the BBQ sauce.
Spread a little barbecue sauce, then a little mustard, evenly on top of each breast. Arrange a bacon slice on each breast, then cover all with cheese.
Bake at 350 degrees F (175 degrees C) for 30 minutes or until chicken is cooked through and juices run clear.