Place 2 of the tortillas, side-by-side, on a cutting board. Layer each tortilla with 1/4 each of the refried beans, beef mixture, black beans, tomato, olives, cheese, avocado, sour cream and green onions, spreading toppings towards the outer edge of tortillas. Top each with a tortilla. Repeat layers, ending with a tortilla. Serve immediately. Or wrap tortes with plastic wrap and refrigerate until ready to serve.