In a medium bowl, combine the chicken, 4 tsp. cornstarch and 1 tbsp. soy sauce. In a small bowl, whisk the remaining cornstarch and soy sauce with the ketchup, sugar, cider vinegar and pineapple juice.
In a large nonstick skillet, heat the oil over medium-high until it shimmers. Add the chicken and cook, turning once, until browned, about 4 minutes. Add the ginger and cook, stirring often, until fragrant, 30 seconds. Add the pineapple chunks and the bell pepper. Cook, stirring occasionally, until the pepper is crisp-tender, about 3 minutes. Stir in the ketchup mixture, bring to a boil and cook until the sauce thickens, about 1 minute.