Imagine if GTA IV never existed and GTA V was released as GTA IV. It would've blown your mind. GTA IV didn't have car customization, golf, animals, pets, tow trucks, moutains, planes, scuba-diving, swimming under water, jet skis, sharks, bicycles, 4-wheelers, buggies... the list goes on and on. Also, in GTA IV, you couldn't save anywhere you wanted, and the controls were sometimes frustratingly clunky for shooting and moving your character around.
GTA V is an enhancement in almost all areas from GTA IV. The only thing that I notice is downgraded is the euphoria engine. The AI doesn't seem as good. In GTA IV, when you hit a person going slow, the person would actually put their hands out to try and protect themselves or someone might stumble a bit, but not fall down. In GTA V, people pretty much fall down no matter what or just turn into ragdoll.