The screenshots that are used for transition animations do not make the dimming "screenshot" effect that occurs when pressing the Menu and Power button. That in

The screenshots that are used for transition animations do not make the dimming "screenshot" effect that occurs when pressing the Menu and Power button. That invisibly occurs immediately prior to an animation, to produce a less computationally intensive 2D effect. The iPhone 4 runs iOS 7 terribly and more likely you are seeing some graphics stuttering from the (albeit limited on that device) new iOS 7 fade/transition animations.