In my opinion, Sega have been quite ok concerning Shenmue since 2015.
They gave Yu the license and we need to remember that there was gonna be a "proper" remaster for S1 & 2 at one point. The only gripe I have is that Sega didn't handle the project internally.
Outsourcing the project to a second-tier Western developer was a questionable decision, perhaps also a "greedy" one, but that's business.
After two years, when it became clear that d3t clearly weren't up to the challenge, Sega's upper management did the only right thing and cut the project down to a no-frills port of the two games.
I think you can't really blame them. S3 didn't really cause enough of a splash in the market and it was not a critically acclaimed game so why would Sega jump at the opportunity to fund S4?
If Yu can find a publisher to fund S4, I can't see Sega standing in the way of the project.