Well, Sylvester Stallone himself responded to “Rambo V” rumours immediately after the poster was spotted at Cannes. Sly himself tells AICN, that he’s definitely done with Rambo for good. However, he says Miramax can feel free to go on without him if they want to continue the franchise.
More on it here:
Does this mean that there will be a Rambo “prequel” or a “reboot” on the way? I would prefer a “prequel” of First Blood, instead of a “remake”. A prequel to “Rambo: First Blood”, the first one would be interesting. They could hire a younger actor to play John Rambo. A younger actor that looks similar to Sly. It be interesting if a Rambo prequel actually shows his days during the Vietnam War. “Rambo V” without Sly starring as Rambo and him not directing wouldn’t work. So this leaves open the possibility of a “prequel” or a “remake” is on the way.
If you think Rambo can’t go on without Sly, maybe it can. “Terminator: Salvation” went on without Arnold being in the lead, and that film was successful. So maybe it can work, never know.