Shenmue 1 and 2 were produced simultaneously and they were able to be released just a year apart that way.
Likewise, Suzuki should be working on both Shenmue 3 and 4 simultaneously right now to ensure the story is concluded.
They already have the team and budget in place.. it's way easier to just hammer out both games now then to start the whole development process all over again in a few years possibly with yet another kickstarter.
Simultaneous development of Shenmue 3 and 4 could be very efficient.
For instance, characters who are slated to appear in both Shenmue 3 and 4 could have all their animation sequences completed for both games.
And when Corey Marshall comes in, they should have him record all his voice work for both games.
They can also take existing environments from Shenmue 3 and redress them for use in Shenmue 4.
Shenmue 1 and 2 really felt like parts 1 and 2 of the same story since they were literally produced at the same time.
There will be a bit of a disconnect going to Shenmue 3 16 years later... but at least Shenmue 3 and 4 would feel like a unified experience if Suzuki were working on them at the same time.