I meant to reply to this, but if you still are looking for players, I'd play. I no longer have Halo 3's campaign, but it is supposed to be coming free to Gold players in the next few months. I have ODST if you need help in that, too.
I don't talk much, so hopefully there would be 2 other people playing who would chat with you. I'm pretty sure I remember where the skulls in Halo 3 are and how to get them, too, so I'd help get those as well.
I've never really played these games in 4 player mode, so that's why I'm curious to help.