In a large skillet heat oil, then add onion and garlic. Saute for 5 minutes. Add zucchini, and saute for another 5 minutes, stirring occasionally. Drain well, and remove skillet from heat. Stir in chicken; set aside.
Preheat oven to 400 degrees F (200 degrees C).
Spray a 10 inch pie plate with cooking spray. Spread 1/2 of the chicken mixture into it, then sprinkle with 1/2 of the cheese. Place 1 tortilla on top of the cheese layer, then spread on 1/2 of the salsa and add 1 more tortilla. Spread the remaining 1/2 of the salsa, then the remaining 1/2 of the chicken mixture over the tortilla. Top with 1 more tortilla and sprinkle with the remaining 1/2 of the cheese.
Cover with foil, and bake in the preheated oven for 40 minutes. Remove cover, and bake for an additional 15 minutes. Let cool for 10 minutes. Cut into wedges, and serve with sour cream.