Why would you want to play GTA IV? Why not the Lost and the Damned; or the Ballad of Gay Tony?
GTA IV has a much longer single-player than either of those however.
Ballad of Gay Tony is still probably worth getting over GTA IV though, because it adds a lot of new things which are a lot of fun, like the APC and several explosive weapons.
GTA IV is still worth playing though. It's not bad at all, and really, I could only recommend that you play all three.