The next GTA game could be virtually anything.
It could be GTA V. Or it could be a new game in the GTA IV era, with related graphics/features/characters (like VC/SA were related to GTA III)
It could be set in Vice City or San Andreas, or even London or Anywhere City. Or it could be somewhere new entirely.
All we have to go on are hints and clues built into GTA IV, some of which will be true, some of which will just be coincidences. From these, most of the signs point to it being in VC, SA or perhaps somewhere in Europe (London/Paris).
I personally think it's most likely to be a new game in the GTA IV era, set in VC, SA or London. I don't think we'll see GTA V until the next generation of consoles (2013-ish)