But it would still feel incredibly rushed, at least in a normal length game (18 hours~) there is simply too many unresolved plot threads to cram in such a short length. With Shenmue 3 barely advancing the story, it's almost like when some people wanting the saga to conclude with the third game.
I'm of the opinion of "give me a proper Shenmue or bust", I'm not clamouring for Suzuki to just conclude the story, I want a good and memorable adventure that doesn't feel in a rush just to end things and be done with it...
That can only happen with either having Shenmue 5, or making Shenmue 4 a MASSIVE game in volume.
EDIT: It could, maybe, possibly fit in 18 hours if the pacing was extremely fast, like a linear action game... Not sure anyone wants that, though? Certainly not me