Sprinkle one-fourth of the arugula, one-fourth of the tomato, one-fourth of the cheese, and one-fourth of the olives over half of the egg. With a spatula, lift and fold the other half of the egg up over the filling. Transfer omelet to a serving plate. (If necessary, wipe out the skillet with a clean paper towel and coat with cooking spray between omelets.) Repeat with remaining egg mixture, arugula, tomato, cheese, and olives to make three more omelets.