Coming Soon! has a rather lengthy piece talking about Rambo IV (Why do they use Roman numerals for sequels, anyway?).
There's no arguing (well, on this site, maybe there is) that Sylvester Stallone is extremely savvy when it comes to the film business and I have a feeling that he could actually make this work. I've seen him recently on the boxing reality show The Contender, and he is still in amazing physical shape. Plus, they're planning on bringing in a younger actor to whom he can pass the "Rambo" baton in case the film is successful enough for continuing sequels.
I won't rehash the details here, but it sounds like it might be interesting. The first Rambo film, although pretty out there (it was a one-hero action film, after all), did make a point about how Vietnam vets were perceived at the time. I think that Stallone probably realizes that the plot for this sort of film has to be a bit more sophisticated than it was 20 years ago to work well.
Source: Coming Soon!