Pour enough four-cheese pasta sauce into the bottom of a casserole dish to cover in a light layer. Arrange chicken breasts into the casserole dish. Pour remaining sauce over the chicken breasts to cover. Cover dish with aluminum foil.
Bake in preheated oven until no longer pink in the center and the juices run clear, 30 to 40 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).
Cover chicken breasts with a layer of cheese and continue baking uncovered until the cheese melts, about 10 minutes more.
Arrange cheese toast onto a baking sheet.
Bake cheese toast in preheated oven until cheese is melted and the toast is browned around th edges, about 10 minutes.
Sandwich each chicken breast between two slices of the toast with the cheesy sides touching the chicken.