Heat gas or charcoal grill. Cut four 18×12-inch pieces of heavy-duty foil; spray with cooking spray.
On one side of each foil piece, place 1 chicken breast. Top each with 1 tablespoon sweet-and-sour sauce and one-fourth of the pineapple, bell pepper and onion. Top with remaining sauce.
Fold foil over chicken and vegetables so edges meet. Seal edges, making tight 1/2-inch fold; fold again. Allow space on sides for heat expansion.
When grill is heated, place packets on gas grill over medium heat or on charcoal grill over medium coals; cover grill. Cook 15 to 20 minutes, rotating packets 1/2 turn after 10 minutes, until juice of chicken is clear when center of thickest part is cut (170°F). Place packets on plates. Cut large X across top of each packet; fold back foil. Top with chow mein noodles.