Bill Murray hasn't agreed to do the movie yet, but Dan Aykroyd said they're filming in the spring regardless. The movie will feature the old cast being, well, too old, so they bring in fresh bodies.
This sounds like a terrible idea. I don't know if that's nostalgia talking, but Dan and Bill made the old movies what they were. I always heard that the more recent game was the "third movie", and there wasn't going to be anymore. At this point I'm thinking that it should stay that way.