A couple of video frames after Kate speaks in the Season 4 Sneak Peek Video, many images flash by. I freeze-framed the video, and saw the following two images. One has someone looking through a broken window, which I think is likely the shack which Ben brought Locke to. The next image was of Christian (Jack’s father) sitting in a chair. The chair looks like Jacob’s chair.
What could this mean? Christian is Jacob?? No, I don’t think that’s likely. I think it’s a lot more probable that Jacob is manifesting himself as Christian.
As for who or what Jacob is, that remains to be seen.