Because the discussion about Niko's nationality I came the the conclusion that GTA IV must be set in 2005.
In 2008 there was the economic crisis and the presidential election, none of this is satirized or mentioned in the game if I remember correctly.
2007 the first Iphone was released, yet it is absent in the game.
The incident where Niko's squad was ambushed did take place 10 years before the events of GTA IV, which was likely in the Croatia war and the fight over Serb Republic of Krajina in 1995 as explained in the other thread. Niko is most likely a Serb from Croatia and his mother still lives there.
The aftermath of 9/11 is largely present on the News, the Patriot Act was to end in 2005, the bridges are closed due to anti terror measures but opened during the game (this is the weakest argument though, but a further hint).
Development of GTA IV began at the end of 2004, it is more likely that the plot is settled during the time of development rather than in a future time, even if it are only a few years.