The two airplanes were part of a three-ship formation flight comprised of an RV-8 airplane in the lead position, an RV-8 in position two, and an RV-4 in position three. The group performed a formation flight briefing prior to flight, followed by three uneventful formation maneuvers after departure. For the fourth maneuver, the lead pilot made a radio call commanding the group to begin a climbing left turn. All members of the group responded in the affirmative, and the lead airplane pitched up and began the turn. After a 2-second-interval, the second RV-8 began the maneuver, following in-trail. The pilot of the RV-4 then began the maneuver, and momentarily lost visual contact with the second RV-8. He then made visual contact with the lead airplane, misidentifying it as the second RV-8. He realized he was in error, and just as he was about to perform evasive maneuvers, his airplane struck the second RV-8. The RV-4 suffered substantial damage to the aft fuselage, with the left wing tip and aileron partially separating from the airplane after making contact with the propeller of the RV-8. The pilot reported that the airplane began a 4,000-foot-per-minute spiraling descent, and did not respond to aileron control inputs. He regained control of the airplane utilizing rudder, elevator, and throttle control, and subsequently performed a forced landing into a field. The RV-8 sustained substantial damage to the outboard section of the right wing leading edge, and returned to the departure airport uneventfully. Both pilots reported no preimpact mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation.