if Shenmue IV was announced I wouldn't imagine that I would be any time soon within the release of Shemue III - I can't see it being teased in the ending with a post credit clip
I imagine there will be quite a gap after the release of S3 and news on S4 - I can't see it being announced at Magic next year, though I'd love to be proven wrong
I see the event of Shemue III as it's own thing, the comeback from Kickstarter, the development and Magic teases to the final release when we finally get our mits on it! But like many here, I'll take anything and look forward to the insight the documentary offers among other backers :)