Prepare the couscous according to package directions, using the butter.
Meanwhile, in a large, heavy skillet, heat 3 tablespoons olive oil over medium-high heat. Add the bell peppers and cook, stirring, until crisp-tender, about 4 minutes. Season lightly with salt and pepper. Transfer to a plate and set aside.
Add the remaining 1 tablespoon olive oil and the tuna cubes to the skillet and cook over medium-high heat, turning with tongs, until no longer pink on the outside, 2 to 3 minutes. Add the reserved bell peppers and cook, tossing, until heated through, 1 to 2 minutes. Sprinkle the parsley on top; season with salt and pepper.
Fluff and mound the couscous onto 4 plates. Spoon the tuna and peppers alongside. Serve with the lemon wedges.