Yeah, the most surprising thing here is that they're jumping straight to IV with a subtitle to boot. Even though this game jumps back in the timeline. My guess is that they want to go with an increasing number every year to drive enthusiasm. But given that this is now outside the "trilogy," why not just go with a spin-off subtitle? I guess they don't want to split the brand into different "types"?
Could this reflect an industry shift? GTAV should basically be San Andreas II, for instance. The Halo side games didn't seem to do much for the brand, also.