Found this on gtav.net
"The title of the new Grand Theft Auto is "Grand Theft Auto V" (or "Grand Theft Auto 5"). Many fans speculated beforehand that it would be called "Grand Theft Auto: Place Name", as titles have taken this form in the past (e.g. with Grand Theft Auto: Vice City and Grand Theft Auto: San Andreas). The reason for this was that Vice City and San Andreas formed a trilogy with GTAIII, and thus were essentially expansions of the GTAIII world. Former Rockstar CEO, Terry Donovan, admitted that this was the plan from the beginning:
It was really important for us to do this trilogy. A fictional New York in a contemporary setting, Miami in the '80s, and a '90s Cali - that was the trilogy. That was the plan from a long time ago.
— Terry Donovan, Former R* CEO (talking to Game Informer)
There was never any indication that the next few games would follow the same pattern. Dan Houser told the Official PlayStation Magazine in 2007 (a year before GTAIV was released) that Rockstar had yet to make up their minds. He said: "to be honest with you, we haven't yet figured out if this [GTA IV] is going to be leading onto another bunch of games or not. We are just focusing on this at the moment."
The announcement of 'Grand Theft Auto V' doesn't necessarily mean that the game won't feature loose connections and returning characters from previous games. It's just a title, afterall."
Doesn't really help i know, but there you go.
It makes me think that 5 could be set in an entirely new place?