Great multiplayer action
The damage system in 'V4' is probably the biggest gameplay change. You actually have a damage bar that is chipped away as you are fired upon by opponents. As your vehicle starts falling apart, your speed and control suffer. Shooting your opponent does't take them out of the race necessarily, but it does put them at enough of a disadvantage to turn the tables on the competition (until they are able to grab a repair powerup).
Other than the adjustment to the weapon system (which can be disabled for those wanting a clean race) the game is basically V3 with some extra polish. There is definitely increased interactivity in the tracks, the graphics are a little nicer, and the HAVOK physics make for some occasional surprises. The 'downfalls' of V3 are also present though, which may make the game frusterating for some. Unless you've played a track before, you'll inevitably get caught in the many traps and pitfalls that are strewn across each track (thanks to the limited camera view). The challenge of avoiding these hazards (and pushing your opponents into them) is part of the fun of V4 in my opinion, and generally makes for many intense multiplayer battles.
The single player mode isn't really worth your time (accept to unlock stuff) as the computer exhibits a supernatural knowledge of the tracks that makes it annoying to race against. Playing against other humans is a quite different experience though. With a group of friends, some beverages, and a common love for old-school partygames you're just about guaranteed a good time.