I played GTA IV in a particular moment in my life, so maybe that situation influenced my response to the game, but seriously... I just have bad memories of GTA IV. I just remember being bored a lot during the game, with just 4 or 5 memorable mission and a character that half of the time is in the background during cut-scenes (like Franlin in GTA V) and a storyline that doesn't really start until half of the game. I remember the disappointment where missions had me to go on internet or play bowling, and the insofference for a gray, dull city with lacks of unique places (most of the buildings are copies from real life or just simple houses... I'm talking about fantasy locations). And I remember the hate for niko, a false backstabber who cry and moan about his past but then kills people whenever someone ask him a favor (that's ok when some boss force him to kill, or when he kills for his own goal, but when an idiot like Brucie or manny ask him to do a dirty job he CAN refuse, if he really want to stop doing stupid things).
GTA IV was the first GTA I stopped playing for some times... I think I'll re-play it sooner or later, but for now I'll go with GTA V, a better game OVERALL
PS: I know I sound harsh, but I really hated GTA IV. I'm open to debate