The overall feel. GTA V might be more light-hearted, less sombre, and more "fun", but it lacked GTA IV's more down to earth, and morally grey world. IV had that sense of urgency, and tells a tale that doesn't go with tropes, and cliche's that separates between good, and evil. IV also felt very connected from start to finish, with every bit of inch from the map heralding involvement with the narrative, and features side missions that do not feel meaningless, but shows a more detailed look how Niko copes, and struggles the American way of life.
GTA IV is a game that is far away from the repetitiveness, and disconnect from most open world games today. GTA V on the other hand embraced the flaws of nearly every single open world game has. It is very impressive at first glance, but you start noticing the disconnect with the story, and the world, the repetition, and the nonsensical approach with the map being too threadbare with hollow side quests scattered through out the game. IV on the most part is very tightly paced, and despite the long 50 hour story it features big, and even small key moments during a playthrough to keep the player intrigued, and excited to prevent boredom of repetition. Instead of adding more to the game for the sake of just adding stuff for more features, just focus on more on what actually matters, and make complete sense to what the game was envisioned.