After the game being leaked on the internet, Rockstar has taken action and authorized retails stores everywhere to start selling the game immediately. 

Can’t wait to get your hands on GTA 4? In the gta iv latest news we’ve just heard that hundreds of Game stores across the UK are opening on Monday evening for you to get your hands on gta4. If you haven’t preorderded yet then get down there ASAP as demand is huge and supplies are short. You’ll be able to pick up the standard Xbox 360 version (with 500 free microsoft points codes) and the PS3 version (with free unlimited multiplayer modes) - however the Grand Theft Auto IV Special Edition is only available via mail-order. One benefit of delivery is that Game have been known to send games out extra early in order to guarantee they meet the official gta4 release date - there will be a lot of happy GTA4 fans out there if this is true. 

 Source