They will have to handle this carefully.
If it's an onward progression from the end of Shenmue III but not a clearly defined part of what could be a Shenmue 4 there will be people baying for blood asking why it wasn't included in the main game.
If it's an episode of Shenmue 4 (or what Shenmue 4 is) then people might be more accepting.
If it's playing Iwao's background in the lead-up this would be a good call for all of us. We don't really know what happened and it would serve as context for all the games.
Anything else is anyones guess, if it is indeed anything. We've had it said to the staff that DLC isn't planned for Shenmue III. So based on that assumption it has to be something seperate.