The SA map I'm not sure about. The GTA4 map however I think is pretty close. You can make out the general size of city blocks and compare them to blocks in the new map, and while the 4 map looks a little shrunk, it's not by much. The airport V shape is also roughly the same size, so it's probably pretty close.
A lot of what looks like empty space to the north, that I'm guessing is the forested sides of mount Chilliad.
I've had this pre-ordered since E3 2012. It was $10 off and another $10 in Best Buy points then. I almost wish I wasn't going away for vacation next week so I could play this but the game will have to wait. It'll be there waiting for me when I get back and I can't see myself doing anything else but playing this on the Sunday I get back.