The ticker on the top of our GTA IV site shows just 11 days remaining until the release of the first trailer, yet we are no closer to knowing where the game will be set. All options are still in the open, and in light of this we have expanded our GTA IV setting page to include New York as a possibility.
London has been a strong candidate ever since GTA 4 was announced, but rumors of New York have grown stronger. The latest case for a New York setting seems to be a topic at NeoGAF forums (topic has since been removed), about a 80's New York-based band called Breakdown being contacted by Rockstar Games for licensing rights to some tracks. Drawing a conclustion between this and GTA4 being set in Liberty City is rather far fetched, yet in any case, the setting page will be refined if the trailer does gives us an indication of GTA 4's setting. Else you're free to keep speculating.