Preheat the oven to 400°F (200°C).
Brush both sides of each corn tortilla with vegetable oil.
Place the tortillas on a baking sheet and bake for 5 to 7 minutes per side or until crispy and golden brown. Remove from oven and set aside.
In a medium bowl, combine the shredded chicken, chili powder, ground cumin, garlic powder, and salt. Mix well to evenly coat the chicken with the spices.
Warm the refried beans in a small saucepan over medium heat until heated through, stirring occasionally.
To assemble each tostada, spread about 1/4 cup of refried beans evenly over each crispy tortilla.
Top the beans with about 1/2 cup of the seasoned shredded chicken.
Add a layer of shredded lettuce and then sprinkle diced tomatoes over the top.
Sprinkle 1/4 cup of shredded cheddar cheese on each tostada.
Add a dollop of sour cream and a spoonful of salsa on each tostada.
Serve immediately while the tostadas are still crispy.