For me what kills it in GTA V is the fact it lacks counters. It doesn't matter whether people think it's clunkier in GTA IV because the counters are more varied and complex.
The melee in Gta 5 is much much worse than GTA 4's. Niko, Luis and even Johnny had some good moves. I don't understand why they would degrade it, it is an essential feature to be more bad-ass than just a guy with a gun, because any fool can pull a trigger.
Niko could disarm knives, baseball bats, deliver roundhouse kicks, uppercuts etc. With GTA V it's just a generic duck and a couple of swings. Boring.
If I had to describe the two GTA IV's is more methodical and reliant on timing. GTA V's is more ADHD like a crappy Bruce Lee simulator.