People need to realize GTA IV was the first Grand Theft Auto title on the new engine. There's no doubt GTA V will have lots of improvements. I'm sure that every game they have been releasing on the RAGE engine is all building up to GTA V. Red Dead Redemption had cover improvements, great countryside, cloth and clothing physics etc. Max Payne 3 is going to have weapon and aiming enhancements, improved AI, better physics and dynamic injuries etc.
I won't be surprised if all these improvements are used in GTA V. GTA IV was just a test for the new engine.