I'm sure the game was at lease 60%-70% finished when they released the trailer. It's just a way of them saying "we're ready to announce the game now." Besides, they said "is in full development" not 'we just started developing", it could have been in development for 4 years and their statement would still be true.
Well, apparently GTA V was being made before IV's release but I'm not that really sure about that. Although, I do believe this game is most likely finished. Rockstar Games just want Max Payne 3 out there first..
I'd say they just started work on it in 2011, with the trailer being a pre-render.
Rockstar never use pre renders. Plus how would all those la setting leaks have come out if they werent in development for at least a year. Just sit back and chill rockstar are working hard to make u happy. These are the same people who made the seried we luv so much