SEGA would need a dramatic change of heart to fund Shenmue IV; it'd have to be driven by the profit generated directly from the Shenmue IP: Shenmue I & II sales, Shenmue III royalties, and merchandising (in the future, the anime). Fan demand and cultural cache has historically meant jack to SEGA when it comes to them fronting their own money.
As of right now, the only thing I would imagine moved the needle for SEGA was Shenmue I & II, which seemed to sell respectably in its first 6 months when factoring in all 3 platforms -- around 350,000, I think? -- and probably has OK legs on PSN, Steam, etc. It wouldn't surprise me if it'd hit 500,000 LTD, albeit with heavy discounts (pure speculation, of course).
But given the I & II re-release probably cost less than half of what IV would cost to fund and publish, is that even enough? Remember Cedric said he was hoping Shenmue III would sell a million copies, in order to make it worthwhile for everyone involved?