We all know that the rear view in GT3 sucked really bad. It looked like it was reflecting GT2. The rear view mirror in GT4 has been fixed - it's pretty much flawless now.
The graphics engine in Prologue was mostly GT3 and the physics were too.
The pop up, slow-down during replays, and other errors are gone. They don't exist. Kaput. Axed. Ironed out. Etc. etc. etc.
GT4 will be the best thing since sliced bread.