R* are legendary at being canny with their projects, simply becasue they are playing with fire. They are trying to make a living breathing environment and that they have, and it's pushing the capabilities of the game engine and platforms.
Yes, the PC port is a bit rough and hardon the hardware and the console versions had similar problems... but so did every other GTA. They all made hardware chug when they first landed and the engines have enver been perfect, as have the stories. I think people are getting a little hung up over nothing... I really enjoyed all the games and they've only ever gone forwards IMO... peoples problem is the gameplay is just more or less the same and they can't be bothered to explore the environment themselves.
It will not be GTA V, it'll be based on the same engine and will just feature a new environment and main character like they did with GTA III and its "expansions". They will be foolish to call it GTA V IMO. They have the oppertunity to bring back Vice City, and TBoGT does hint quite wildy towards it in places. I really hope it is, it will be stunning in the IV engine.
Whatever they do, we won't hear or see anything until they are happy to show it, and it won't be renders or impressions either. R* market GTA very well and people will rush out and buy it at midnight like I did with IV, but I hope they take the oppertunity to tweak the engine to keep the pedants happy.