What size is the RV-4? Normally for a small aircraft its not worth the effort for the amount of performance unless you are trying to go scale. Otherwise a 40 size aircraft can benefit from flaps, its not a necessity but a "neat" thing to have. There are advantages of flaps over flaperons. For instance, flaperons increase the angle of incidence over the entire span slowing the airplane but at the same time reducing the aileron effectiveness at the tips where is needed the most. While flaps only increase the incidence at the span section of the flap, leaving the wing tip incidence intact with plenty of aileron effectiveness. Problems with flaps? Its twice as much work as having just flaperons and more material and hardware making the aircraft heavier. Bottomline, its a personal choice.