Season tilapia fillets on both sides with salt and pepper.
Mix 3 tablespoons flour and garlic powder together in a shallow bowl. Dredge tilapia fillets in flour mixture; shake off excess.
Melt 3 tablespoons margarine in a large skillet over medium-high heat. Cook fillets in the hot margarine for 2 minutes. Flip; cook until crust is crisp and flesh just starts to flake when pressed with a fork, about 1 minute more. Transfer fillets to a paper towel-lined plate.
Melt remaining 1 tablespoon margarine in the same skillet over medium heat. Scrape the browned bits of fish off the bottom of the skillet with a wooden spoon. Stir in remaining 1 tablespoon flour until thickened. Add lemon juice and capers.
Pour fish stock into the skillet; cook, stirring frequently, until reduced by half, 5 to 10 minutes. Return fillets to the skillet and spoon reduced stock on top. Garnish with parsley.