I really cant rap my head around the bias GTA4 fanboys have - saying GTA4's driving mechanics are much better than GTA5's, that GTA4 has better handling and is much more realistic (almost employing its a simulator) and that GTA5 is dumbed down arcady i.e. went downhill (like they are completely different, black and white).
First they are both very similar, they are both built off the same engine, they are both VERY!! arcady, and both are VERY!! far from realistic.
The only major difference is that in GTA4's world cars completely understeer and the suspension is made out of jelly so cars can just flip over like they are on the moon. In GTA5's world cars completely oversteer and the suspension is so good that cars can take turn super faster and have more downforce than an F1 car.
The problem with the driving mechanics in both games is the same problem that the on-foot mechanics of both games have (where you fall down if you tap of anything) - The Euphora animation engine.
I think driving mechanics more alike to Need for Speed Rivals, Midnight Club LA, Burnout Paradise etc..., or even Driver San Francisco would be much better than what we already got with GTA4 and GTA5.