RV-4 Pilot reported a failure of the aft seat during high G aerobatic maneuvers caused the Flight Instructor sitting in the seat to fall backwards into the baggage compartment. Upon review of the aircraft logbooks; it is suspected a Service Bulletin regarding rear seat reinforcement may not have been complied with.

Narrative

I did an hour of aerobatics and received my spin endorsement in a biplane with an Aerobatics Instructor. I later flew to him for further aerobatics instruction in my RV-4. We executed various aerobatic maneuvers including loops; pulling approximately 3-4 Gs.At the end of the lesson; my Instructor tested my upset recovery skills by putting me in an unusual attitude. I corrected too aggressively and pulled 6.5 Gs. As I did so; the rear seat collapsed; sending my Instructor into the baggage compartment. Fortunately my Instructor incurred only minor injuries; but he could have sustained severe spinal damage or; if he had been gripping the stick at the time; could have yanked back hard enough to rip the wings off.I believe the main cause was that the rear seat was not in compliance with SB 97-05-1 'Rear Seat Reinforcement'. My last condition inspection states 'carried out an annual inspection per Vans condition inspection checklist' and Van's Condition Inspection Checklist states 'Review Service Bulletins'; yet I was not informed; nor was it logged; that there were any SBs not complied with.A contributing factor is that I pulled more Gs than planned due to my inexperience with aerobatics; fear of overspeeding the plane; and an aft CG that is known to make the RV-4 sensitive to back pressure. It is possible that the 0.5 over-G contributed to the seat back failure or that the seat back failure contributed to the over-G. W&B at the time of the incident was 1;359 pounds and 65.9 inches CG. Aerobatic limits are 1;375 pounds and 65.9 inches CG.The plan is to apply SB 97-05-1 'Rear Seat Reinforcement'; check compliance with all other SBs; and perform a thorough condition inspection.