About fifty chapters of tweaks between the 2003 and 2004 model Prius documented in the New Car Features manual. Some chapters have dozens of tweaks and some only have a couple. I never counted the actual number but around 2,000 would be my guess. And lights, seats, motors, safety devices, and suspension, HVAC, the list goes on. I don't think there is even one part in common, other than some fasteners and maybe the horn, between the 2003 and 2004. I purchased a 2001 Prius for Denise and she still loves that car, so I reckon I can call myself an early adopter.