From what I've seen of RDR it's basically GTA4 in the west. This definitely means that the crux of GTA's main problem right now is setting. I don't even think a different country/continent would fix things. It has to be a completely different time period.
You couldn't be more wrong if you tried. You're positing that the only difference is the setting, when in fact the larger divergence between GTA IV and RDR exists in the interactivity between the player and the world.
GTA IV is a beautiful, detailed world, but nothing happens in it unless you're on a mission. You can DO A STORY MISSION or you can GO RACE or you can CALL A DATE and do ONE OF FOUR BORING ACTIVITIES TOGETHER. Or sometimes THEY CALL YOU, but if you refuse to go out with them it reduces your friendship rating with that character. Whenever you , you have to TRAVEL to pick them up, sometimes all the way across the map, then TRAVEL to the location, then do something boring like DARTS or POOL, and then TRAVEL back to their place.
In contrast the interaction with the townspeople in RDR is immediate: someone has been robbed, so go get the thief; someone's horse was stolen, so go retrieve it; someone is kidnapping a victim or going to knife them, so save them; someone wants to duel... It's all right there, right in front of you, and you don't have to go looking for it.
And those are just the in-town instances; let's talk about being out looking for specific environmental objects like herbs or flowers and suddenly being confronted by something that doesn't like you being there, like a grizzly or a cougar. Or how about stopping to help someone and being ambushed by a half dozen heavily armed jerks?
All of these encounters could be transposed into the GTA IV world and made it deeper and more enjoyable. It's been stated repeatedly in interviews with Rockstar that they wanted to tell an immigrant's story, and they wanted the player to be able to focus on that. It follows that they removed the simple R3 Missions from GTA3 series games because of that goal. RDR has shown that with enough creativity you can keep the spontaneous-appearing events without impairing the overall narrative. You'll be seeing similar, even analogous activities in GTA V, for certain.
