The 4th trailer was announced to a German website cynamite.de. Also, the website claims that Rockstar also said they will reveal in-game clips AFTER the 4th trailer is shown to public. Take this news how ever way you do however, but this site looks much too credible to be bogus.
"GTA IV: Rockstar Games announces fourth trailer
Now official: A fourth trailer of Grand Theft Auto 4 will come.
Fourth GTA IV trailer.
A spokesman for Rockstar Games has over Cynamite confirmed that a fourth of GTA IV trailer gets. The trailer, like the previous ones, in the film's trailer-style "be kept and no direct game scenes. A video with live action from GTA IV to be until after the fourth Trailer published. 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."
Click here for the source, in German. Use Babelfish of Google Translate for help.