Spread 1/4 of the beans over each tostada shell, top with 1/4 of the barbacoa and 1/4 of the cheese. Bake for 8-10 minutes until the cheese is melted and the shell is crispy.
Top each tostada with 1/4 of the lettuce, pico de gallo, and sour cream. Garnish with freshly chopped cilantro. Serve immediately!