To prepare the quesadillas, in a saute' pan or skillet over medium-high heat, melt the butter. Add the mushrooms and saute' for about 3 minutes, or until golden brown. Spread 1/2 tablespoon of the cheese on one side of each tortilla. Place about 1/4 cup of the turkey evenly over the cheese on 1 of the tortillas. sprinkle 2 tablespoons of the cooked mushrooms over the turkey and top with another tortilla, cheese side down. Keep warm while preparing the remaining 3 quesadillas.