According to Wikipedia (yes, I know they aren't always 100% but its still a good read with some potentially good info!):
"On February 2, 2008, Sylvester Stallone informed Reuters in an interview that making a fifth Rambo would depend on the success of the fourth movie but that he thought he was "gearing one up" and that it would "be quite different".[33] On March 10, 2008, Moviehole.net revealed that in an interview with Metro magazine, Stallone said that he was "half-way through" writing Rambo V and that it would not be another war movie.[34] On March 20, Moviehole.net also said that ScreenDaily.com had revealed that the film would be shot in Bulgaria and later received word from a source that Bulgaria was to double for John Rambo's home town in Arizona.[35][36] On September 9, Moviehole.net reported that Stallone had finished writing Rambo V and will be directing it. He is also set to write Rambo VI as well.[37] On November 3, 2008, Stallone confirmed that they will be filming a Rambo V and that it's in the works. Stallone said: "Rambo will be back. We've just got to give you guys a story that's worthwhile." It is rumoured that the new film's plot (which was an alternative for Rambo) involves Rambo's unknown daughter being kidnapped by cult leaders and he goes on a mission to rescue her.[38] On February 4, 2009, Sylvester Stallone quoted, "Yeah, we are doing another 'Rambo,' but the conflict is whether to do it in America or a foreign country".[39] Initially, the plot of "Rambo 5" appeared to be centered around Rambo being hired to hunt down a violent, genetically engineered creature that is running loose in the Arctic Circle. [40] Later it was decided that the plot would be used for another movie unrelated to the Rambo franchise."
http://en.wikipedia.org/wiki/Rambo_(film)#Sequel