Longer, better written and more meaningful story. Characters to like and actually care about. Cars actually get deformed when crashing. Better selection of cars (more sedans and coupes, with tuner variants) and more diverse controls. Motorcycles are actually useful in IV, and more fun than V for that matter. V has way too much super cars and the cars all feel the same, they all drive as sports cars. A wooden AK and better gun selection. Combat Pistol in IV was a Desert Eagle, in V it's a USP or something. Better gun sounds. No military and government interactions, except for the UL Paper contact - but he wasn't exactly making you do Call of Duty-esque missions (i.e. GTA V's "Monkey Business") and he actually helps you in the end. More realistic ragdoll and pedestrian damage effects (they can limp away after beaten too hard and hold their wounds after being shot too much compared to V where they just die.). Better melee combat system. Better map (V is bigger, not better. Don't confuse the two). More minigames, such as pool and bowling, more friends to hangout and more meaningful conversations with them. More interiors and an actual eating option. No health regeneration. I prefer simple cops over terminator cops, so the cop system is better in IV. Better soundtrack and Lazlow was actually funny. Richard Bastion was pretty good too. More side-jobs. An online mode more focused on fun and less on milking their fans for money.
Honestly, the only thing V does better than IV without question is graphics... V came out 5 years after IV, so OBVIOUSLY it's going to.
Every. Single. Other thing. Is arguably (remember; arguably) better in IV than it is in V.