Yeah, I don't think that's a real movie poster, but a graphic designer's (and a pretty good one, at that) idea for a GFIV poster. If you go to the main site it has redone poster's of GFI and GFII and says "studio redesigns"
It's a fake I guess. Just look at the names below the picture of Andy Garcia, his name isn't even there. I do see the name of Al Pacino there though, don't know from which movie that part of the poster comes.
One name that was on there is Naomi Watts. I dont know what the rest of you think, but I don't know why one of the main characters would be Naomi Watts. She's a good actress, just unsure about her in the Godfather.