Maybe this has already been covered....But shouldnt it just be called "GTA IV San Andreas" ? since id imagine its the same game engine, just in a different city. Thats how they been titled them so far. The actual number isnt change till the whole game is revamped
This was my initial thought but then i realised something. You had the GTA 1 series, containing the three cities and a GTA london. Then the engine changed to GTA2, but there wasn't a GTA2 series, just GTA2 as a stand alone game. Next we had the GTA3 series, with the 3 cities on the GTA3 engine. After this we got GTA4 which was a new engine but not a series, just like GTA2 it's one stand alone game.
Now we have GTA5 which is said to be a new direction for GTA and bigger than anything previous, so perhaps we will get a GTA5 series with a bunch of cities running on the new engine.
Or maybe I'm wrong and it's just gonna go GTA5, GTA6, GTA7 from now on...