Metal Gear Solid 4 is grabbing every previously seen character that's still alive and making sure they've got a place in the new game. Take for example these two ladies, Mei Ling and Rosemary, who save your game via codec call from MGS1 and MGS2 respectively. What their roles are exactly in MGS4, we don't know. It's unlikely Rose would reprise her role as game saver, seeing as Raiden is most likely non-playable. Unless, Snake shoots himself going in one-fourth into the game and you'll end up playing as Raiden for the rest of the way. That would be an absolute riot. Of course it's not going to happen though.