Yeah, 'complete redesign' seems a little effusive. Somebody read the a journal article about post-cat EGR, which had been a subject of active research industry-wide during the Gen 3 years but hadn't been ready for prime time yet. Then they took the dimensions of the exact EGR system they already had, plugged them into the flow equations with the lower exhaust pressure after the cat, and made the new system the right amount larger. About the only thing that seems 'redesigned' in the later system is the coolant nipples added to the EGR valve itself. As far as I can tell, they first added those to the Prius c in 2012.