German gaming website, Cynamite.de (link translates to English), posted earlier today that an unnamed spokesman for Rockstar Games has announced a fourth Grand Theft Auto IV trailer. The site also suggests the final release date to be in the April time frame, one month after what has already been suggested by others.

"A spokesman for Rockstar Games has over Cynamite confirmed that a fourth of GTA IV trailer gets. Der Trailer soll, wie die vorherigen, im "Filmstrailer-Style" gehalten sein und keine direkten Spielszenen zeigen. The trailer, like the previous ones, in the film's trailer-style "be kept and no direct game scenes. Ein Video mit Live-Action aus GTA IV soll erst nach dem vierten Trailer veröffentlicht werden. A video with live action from GTA IV to be until after the fourth Trailer published.
Zu den Veröffentlichungsterminen der neuen Videos hält sich Rockstar Games allerdings genauso bedeckt wie zum Releasetermin des fertigen Spiels. The release dates of new videos adheres Rockstar Games, however, as cloudy as the release date of the final game. Wir gehen weiterhin von einer Veröffentlichung im April 2008 aus. We continue to go from one publication in April 2008." -- Cynamite.de

Note that this is all rumor though, nothing official yet.