Cut chicken into bite-size pieces. In small bowl, mix chicken and salsa. Spray 1 side of 1 tortilla with cooking spray; place sprayed side down in 10-inch nonstick skillet. Layer with one-fourth of the chicken mixture and 1/2 cup of the cheese. Top with another tortilla; spray top of tortilla with cooking spray. Cook uncovered over medium heat 4 to 6 minutes, carefully turning after 2 minutes, until golden brown. Repeat with remaining tortillas, chicken mixture and cheese. To serve, cut quesadillas into wedges. Serve with sour cream and, if desired, additional salsa.