I've got something that is based on this
thread witch I've put together.
1. TBoGT has a big similarity in missions and storyline as San Andreas and Vice City
2. The shredded piece out of the poster show's a part of a other poster witch has been showed in San Andreas and Vice City.
Maybe a possible Conclusion: The poster says something about the TBoGT story and/or that the next GTA will be set in SA or VC.
It says not much but maybe it clears a little bit up.
Have you noticed some of the dialogue's in TBoGT and IV? If you hear them you think R* already gave the location away. For example: if you do the club management side missions you'll be called to pick some Al guy up at Hercules (it's not the rapper),He will say in the dialogue's with Luis ''Why aren't you expanding to Vice?'' and ''Vice is better than that Los Santos crap''.
If you'll count the time ''going to Vice'' is said on radio's and in some dialogue's it looks like the focus is set on Vice City.
Again: This is only a suggestion of things I noticed and should be possible.