Heat gas or charcoal grill. Cut four 18x12 in pieces of heavy duty foil; spray with cooking spray
On one side of each foil piece, place 1 hicken breat. Top each with 1 tablespoon swwet and sour sauce and one fourth of the pineapple, bell pepper and onion. Top with remaining sauce. Fold foild over chicken and vegetables so edges meat. Seal edges, making tight 1/2 in 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 15-20 minutes, rotating packets 1/2 turn after 10 minutes, until juice of chicken is clear when center of thickeest part is cut (170 F).
Place packets on plates. Cut large X across top of each packet; fold back foil. Enjoy!